Hi,
sorry I need to switch to English language for a longer post.

book, bookb and bookw files must have been corrupted somehow in JA's package.
Indeed all 3 files are required to be present.

For testing purpose, I built a very small book with only few unusual moves to substitute it to the provided book(s) and I can point out:
- Cray Blitz playing black uses "bookb" first, then "book" if end of book is reached
- same process, playing white it uses "bookw" first then "book".

Re-building the main "book" file was therefore enough: as neither initial "bookw" nor "bookb" are able to provide any book move, Cray Blitz uses the rebuilt "book" only.
One can copy the rebuilt "book" to bookb and bookw, these will be used but of course without any change to the openings played.
One can as well build an empty book to replace current bookb and bookw, result is the same.

The good news is one can easily enforce a specific line of opening, building the relevant tiny bookb or bookw with only few desired starting moves. Then Cray Blitz will still use its full book for next moves...

Schönen Gruß,
Tibono