For me it’s mot clear at all what the difference between “Book” and “Digibook” is.
As described on https://musicbrainz.org/doc/Release/Packaging it’s very similar to me.
In difference to “Digipak” (compare https://en.wikipedia.org/wiki/Optical_disc_packaging#Digipak_/_Digipack) I think there is no trademark on “Digibook”.
BTW: “Digipak” will be used more in a generic Digipack here, I’ll guess.
Back to Book/Digibook.
Book: “A book with a sleeve containing a medium (usually a CD).”
Digibook: “A bounded booklet usually in hardcover with a sleeve bound to the spine of the book that houses a CD.”
What is the difference between a book and a bounded booklet? For me it’s more or less the same (or it could be more or less the same).
Also the picture for Book (https://wiki.musicbrainz.org/-/images/a/a5/Packaging-Book2CD.jpg) fits to Book and Digibook to me.
An example: The packaging for release https://www.discogs.com/Cecilia-Bartoli-Les-Musiciens-Du-Louvre-Marc-Minkowski-Opera-Proibita/release/1108192 (Cecilia Bartoli - Opera Proibita: “Housed in a 64-page digibook”) is often called a Digibook. I would also use rather Digibook but it looks very similar to the picture used for book (CD in hands).
If the format is not around 142 x 125 mm I would typically use Book.
Maybe the picture used for Book is not the best. Maybe it would be fine to have also a picture for Digibook.