A 'book page' is a page of content that will automatically display navigational links at the bottom of the page that lead to adjacent pages of the same 'book'.
It provides another level of navigation for reviewing related content. Book pages also provide a link to a printer-friendly version of the page. If the 'book page' is the first page of a book, it will by default also have a list of links at the bottom that represents each page in the book.
Figure below: First page of a book
Figure below: One of the pages in the above book