Books are combined with Comics and Magazines into a single Realm named Comics & Books.
Comics and Magazines are relatively easy to separate into "Titles" with multiple "Issues" but Books don't arrange themselves quite so easily. The separation of books much less clear. In fact, you will find many different examples of how a books have been grouped into "Titles" to try and match the comic structure.
The choice is made very much on a case-by-case basis, according to where the cleanest separation can be found between logical groupings.
The one definite guideline is the difference between "Book" and "Color/Activity". A book is "Color/Activity" when performing the activity causes permanent visual damage to the book. Hence a book with audio press-buttons is just a Book, and a Jigsaw is also a Book. But if you have to make a mark on the page, or remove a sticker, then that becomes Color/Activity.