There are at least two main reasons why domestic cats sleep on books. The first is because it is a cooler surface than the surrounding surfaces. Your cat wants to be cooler for whatever reason.
The second reason is for the purposes of scent exchange. Your scent is on the book and he wants to merge his scent with yours for the purposes of reassurance. He feels better emotionally.
A third possible reason is to get your attention by stopping you using the book.