between the pages of a book
Refers to something being placed or found inside a book, between its pages.
I put a bookmark between the pages of the book.
between two books
Refers to the space or gap between two separate books, such as on a shelf.
Dust has accumulated between the books.
Phrase composed of 本 (book) + の (possessive particle) + 間 (space, gap). The meaning depends on whether 本 is interpreted as a single book or multiple books.