It really depends on the book for me. Technical topics I already know pretty well, I find easy to commit new methods to memory when I come across them. Nontechnical, I can remember a good deal of fiction, but factual things fall off pretty quickly.
I think the most important thing is chunking[1]. You remember key things, such as storylines or plots from fiction, or, with technical writing, where to return if you find yourself needing a particular concept in practice.