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.
“Book” is for… books. The picture currently listed for Book seems to be a Digibook and not a book at all. E.g., Absolute Beginner’s Bodhrán Tutor and Rivstart A1+A2 are (soft-cover) books. They’re books that come with a CD.
A Digipak® (trademarked term) or digipack (generic term) consists of a rectangular cardboard package with one or more plastic trays capable of holding a disc attached to the inside. There are variations where the discs sit on a hub or spindle inside and come in various sizes.
Digibook
A digibook (a.k.a. mediabook) is a type of packaging that has a hard cover (like a hardbound book) and comes in various sizes. The disc can either slide into the package or sit on a spindle, hub or tray inside.
Artbook
An artbook is a type of packaging that has a hard cover (like a hardbound book) and is normally around 11 1/4" to 11 1/2" square and contains a several-page booklet inside. Basically, it’s an oversize deluxe digibook/mediabook.
Their definition of Digibook includes different ways of holding the CD. Though their Artbook is much bigger than the Book definition at MB. I would say the Radiohead example I posted earlier is certainly a Book.