From https://github.com/skodak/moodle-mod_book/issues/23

> I have a problem with Importing Book between Courses. Import does not copy book files into new course, so the links to that files are broken.

> Hello, what moodle version, what book version, where are the files stored exactly?

> Moodle version 2.2, and book version - current MOODLE_22_STABLE,
Files are stored in course files.

So this should be verified and fixed, if necessary, for 2.3 as a minimum