raphi011 · raphi011 · Jun 28, 2026 · Jun 28, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -2,7 +2,14 @@
 
 All notable changes to this project will be documented in this file. Take a look at [the migration guide](docs/Migration%20Guide.md) to upgrade between two major versions.
 
-<!-- ## [Unreleased] -->
+## [Unreleased]
+
+### Added
+
+#### Shared
+
+* The EPUB parser now exposes the publication's ISBNs through the new `metadata.isbns` property. ISBNs are read from the OPF `dc:identifier` elements, recognized via an `opf:scheme="ISBN"` attribute (EPUB 2), an `identifier-type` ONIX code (EPUB 3), a `urn:isbn:` URN, or Calibre's `isbn:` value prefix. Each value is returned as declared (scheme prefix and separators removed); a publication may expose more than one (e.g. its ISBN-10 and ISBN-13).
+
 
 ## [3.10.0] - 2026-06-24
 

diff --git a/Sources/Shared/Publication/Extensions/EPUB/Metadata+EPUB.swift b/Sources/Shared/Publication/Extensions/EPUB/Metadata+EPUB.swift
@@ -7,10 +7,29 @@
 import Foundation
 
 private let mediaOverlayKey = "mediaOverlay"
+private let isbnKey = "isbn"
 
 public extension Metadata {
     /// Media overlay CSS class names for this publication.
     var mediaOverlay: EPUBMediaOverlay? {
         try? otherMetadata[mediaOverlayKey]?.decode()
     }
+
+    /// ISBNs declared by the publication, as found in the OPF.
+    ///
+    /// The package's primary `identifier` is usually a UUID, so an ISBN — when
+    /// present — is typically a *secondary* `dc:identifier` tagged via an
+    /// `opf:scheme` attribute (EPUB 2), an `identifier-type` ONIX code (EPUB 3),
+    /// or expressed as a `urn:isbn:` URN. A publication may declare more than one
+    /// (e.g. the ISBN-10 and ISBN-13 of the same book, or distinct ISBNs per
+    /// format). Values are returned as declared, with only the scheme prefix and
+    /// separators removed — no normalization. Empty when the publication carries
+    /// no recognizable ISBN.
+    var isbns: [String] {
+        guard case let .array(values)? = otherMetadata[isbnKey] else { return [] }
+        return values.compactMap {
+            guard case let .string(value) = $0 else { return nil }
+            return value
+        }
+    }
 }
diff --git a/Sources/Streamer/Parser/EPUB/EPUBMetadataParser.swift b/Sources/Streamer/Parser/EPUB/EPUBMetadataParser.swift
@@ -41,6 +41,7 @@ final class EPUBMetadataParser: Loggable {
 
         var other = metas.otherMetadata
         if let mo = mediaOverlay() { other["mediaOverlay"] = .object(mo.jsonObject) }
+        if !isbns.isEmpty { other["isbn"] = .array(isbns.map { .string($0) }) }
 
         return Metadata(
             identifier: uniqueIdentifier,
@@ -323,6 +324,49 @@ final class EPUBMetadataParser: Loggable {
         dcElement(tag: "identifier[@id=/opf:package/@unique-identifier]")?
             .stringValue
 
+    /// The publication's ISBNs, as declared in the OPF.
+    ///
+    /// The package `unique-identifier` is usually a UUID, so an ISBN — when
+    /// present — is a *secondary* `dc:identifier` tagged via an `opf:scheme`
+    /// attribute (EPUB 2), an `identifier-type` ONIX Code List 5 refinement
+    /// (EPUB 3), or expressed as a `urn:isbn:` URN. A publication may declare
+    /// more than one (e.g. the ISBN-10 and ISBN-13 of the same book, or distinct
+    /// ISBNs per format), so every matching `dc:identifier` is returned, in
+    /// document order with duplicates removed. Values are surfaced as declared,
+    /// with only the scheme prefix and separators stripped — no normalization.
+    private lazy var isbns: [String] = {
+        var seen = Set<String>()
+        return metas["identifier", in: .dcterms]
+            .filter { isISBNIdentifier($0) }
+            .compactMap { Self.cleanedISBN($0.content) }
+            .filter { seen.insert($0).inserted }
+    }()
+
+    private func isISBNIdentifier(_ meta: OPFMeta) -> Bool {
+        // EPUB 3 (spec) tags the scheme with an `identifier-type` meta refining
+        // the `dc:identifier`, carrying an ONIX Code List 5 value (15 = ISBN-13,
+        // 02 = ISBN-10). EPUB 2 uses an `opf:scheme` attribute on the element.
+        // Calibre's EPUB 3 writer and the URN form instead prefix the value
+        // text (`isbn:…` / `urn:isbn:…`).
+        let value = meta.content.lowercased()
+        if value.hasPrefix("urn:isbn:") || value.hasPrefix("isbn:") { return true }
+        if meta.element.attr("scheme")?.caseInsensitiveCompare("isbn") == .orderedSame { return true }
+        let onixCode = meta.id.flatMap { metas["identifier-type", refining: $0].first?.content }
+        return onixCode == "15" || onixCode == "02"
+    }
+
+    /// Strips the scheme prefix (`urn:isbn:` / `isbn:`) and separators from a raw
+    /// identifier value, returning the bare ISBN as declared. nil when empty.
+    private static func cleanedISBN(_ raw: String) -> String? {
+        var value = raw.trimmingCharacters(in: .whitespacesAndNewlines)
+        for prefix in ["urn:isbn:", "isbn:"] where value.lowercased().hasPrefix(prefix) {
+            value = String(value.dropFirst(prefix.count))
+            break
+        }
+        value = value.filter { !$0.isWhitespace && $0 != "-" }
+        return value.isEmpty ? nil : value
+    }
+
     /// https://github.com/readium/architecture/blob/master/streamer/parser/metadata.md#publication-date
     private lazy var publishedDate =
         dcElement(tag: "date[not(@opf:event) or @opf:event='publication']")?

diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-calibre.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-calibre.opf
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>Programming Scala</dc:title>
+    <dc:identifier id="pub-id">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier>isbn:9781449325862</dc:identifier>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-duplicate.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-duplicate.opf
@@ -0,0 +1,16 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>Programming Scala</dc:title>
+    <dc:identifier id="pub-id">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier opf:scheme="ISBN">9781449325862</dc:identifier>
+    <dc:identifier>urn:isbn:9781449325862</dc:identifier>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-epub2.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-epub2.opf
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="2.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>Programming Scala</dc:title>
+    <dc:identifier id="pub-id" opf:scheme="UUID">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier opf:scheme="ISBN">978-1-4493-2586-2</dc:identifier>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-epub3-isbn10.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-epub3-isbn10.opf
@@ -0,0 +1,16 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>The C Programming Language</dc:title>
+    <dc:identifier id="pub-id">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier id="isbn-id">0306406152</dc:identifier>
+    <meta refines="#isbn-id" property="identifier-type" scheme="onix:codelist5">02</meta>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-epub3.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-epub3.opf
@@ -0,0 +1,16 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>Programming Scala</dc:title>
+    <dc:identifier id="pub-id">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier id="isbn-id">9781449325862</dc:identifier>
+    <meta refines="#isbn-id" property="identifier-type" scheme="onix:codelist5">15</meta>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-multiple.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-multiple.opf
@@ -0,0 +1,16 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="2.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>Programming Scala</dc:title>
+    <dc:identifier id="pub-id" opf:scheme="UUID">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier opf:scheme="ISBN">9781449325862</dc:identifier>
+    <dc:identifier opf:scheme="ISBN">1449325866</dc:identifier>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-urn.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn-urn.opf
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="3.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>Programming Scala</dc:title>
+    <dc:identifier id="pub-id">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier>urn:isbn:9781449325862</dc:identifier>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Fixtures/OPF/identifier-isbn10.opf b/Tests/StreamerTests/Fixtures/OPF/identifier-isbn10.opf
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="pub-id" version="2.0">
+  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
+      xmlns:opf="http://www.idpf.org/2007/opf">
+    <dc:title>The C Programming Language</dc:title>
+    <dc:identifier id="pub-id">urn:uuid:7408D53A-5383-40AA-8078-5256C872AE41</dc:identifier>
+    <dc:identifier opf:scheme="ISBN">0306406152</dc:identifier>
+  </metadata>
+  <manifest>
+    <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml" />
+  </manifest>
+  <spine>
+    <itemref idref="titlepage"/>
+  </spine>
+</package>
diff --git a/Tests/StreamerTests/Parser/EPUB/EPUBMetadataParserTests.swift b/Tests/StreamerTests/Parser/EPUB/EPUBMetadataParserTests.swift
@@ -393,6 +393,60 @@ class EPUBMetadataParserTests: XCTestCase {
         XCTAssertNotNil(sut.otherMetadata["mediaOverlay"])
     }
 
+    // MARK: - ISBN
+
+    func testParseISBN13FromEPUB3IdentifierType() throws {
+        let sut = try parseMetadata("identifier-isbn-epub3")
+        XCTAssertEqual(sut.isbns, ["9781449325862"])
+    }
+
+    func testParseISBNFromEPUB2OPFScheme() throws {
+        let sut = try parseMetadata("identifier-isbn-epub2")
+        // Hyphens stripped from "978-1-4493-2586-2".
+        XCTAssertEqual(sut.isbns, ["9781449325862"])
+    }
+
+    func testParseISBNFromURN() throws {
+        let sut = try parseMetadata("identifier-isbn-urn")
+        XCTAssertEqual(sut.isbns, ["9781449325862"])
+    }
+
+    /// ISBN-10 values are returned as declared, not converted to ISBN-13.
+    func testParseISBN10FromEPUB2OPFScheme() throws {
+        let sut = try parseMetadata("identifier-isbn10")
+        XCTAssertEqual(sut.isbns, ["0306406152"])
+    }
+
+    /// Calibre's EPUB 3 writer prefixes the scheme into the value text rather
+    /// than using the spec's `identifier-type` refinement.
+    func testParseISBNFromCalibreSchemePrefix() throws {
+        let sut = try parseMetadata("identifier-isbn-calibre")
+        XCTAssertEqual(sut.isbns, ["9781449325862"])
+    }
+
+    func testParseISBN10FromEPUB3IdentifierType() throws {
+        let sut = try parseMetadata("identifier-isbn-epub3-isbn10")
+        XCTAssertEqual(sut.isbns, ["0306406152"])
+    }
+
+    /// A publication may declare several ISBNs (e.g. ISBN-13 and ISBN-10); all
+    /// are returned, in document order.
+    func testParseMultipleISBNs() throws {
+        let sut = try parseMetadata("identifier-isbn-multiple")
+        XCTAssertEqual(sut.isbns, ["9781449325862", "1449325866"])
+    }
+
+    /// The same ISBN declared under more than one scheme is returned once.
+    func testParseDuplicateISBNsAreDeduplicated() throws {
+        let sut = try parseMetadata("identifier-isbn-duplicate")
+        XCTAssertEqual(sut.isbns, ["9781449325862"])
+    }
+
+    func testNoISBNWhenOnlyUUIDIdentifiers() throws {
+        let sut = try parseMetadata("identifier-unique")
+        XCTAssertEqual(sut.isbns, [])
+    }
+
     // MARK: - Toolkit
 
     func parseMetadata(_ name: String, displayOptions: String? = nil) throws -> Metadata {