As far as I know, the official release artifacts (e.g. rust-1.9.0-x86_64-unknown-linux-gnu.tar.gz) contain rustc, cargo and rustdoc binaries. Why don't we add rustbook binary as well?
I have been working on Japanese translation of The Rust Programming Language and it requires rustbook binary to build HTML files. On my CI environment, I am using rustbook crate, which is basically a copy of tools/rustbook in this repository (rust-lang/rust).
It is not too bad but it is tedious to keep the rustbook crate up-to-date to the original in this repository. It would be much easier to me and other people using rustbook if the official release artifacts contain it.