expression
between the pages of a book
Refers to something being placed or found inside a book, between its pages.
しおりを本の間に挟んだ。
I put a bookmark between the pages of the book.
expression
between two books
Refers to the space or gap between two separate books, such as on a shelf.
本の間にほこりがたまっている。
Dust has accumulated between the books.