Import books as chapters

[Acclorite]

Why you want this feature to be implemented? Please describe.
Would be great to allow importing multiple files as chapters of one book and not only as standalone books.
Let's say that user has a folder "Cool Book" with 5 ".txt" files and wants to import it as a single book with 5 chapters.

Describe the feature you'd like to be implemented
A way to import books as chapters instead of separate books. Also provide a way to extend that book later (by adding files as chapters once more).