You can retain more of the knowledge you glean from books by prepping your brain with background information before reading. That makes Wikipedia a great tool for readers.
In a post about becoming a faster reader, entrepreneur Ryan Battles explains:
There is a relationship between background knowledge and reading comprehension. The more you understand about a particular subject, the more "hooks" keep the facts in there. So if you are going to read a book on a subject you don't know much about, check out the Wikipedia article on it first to prep your brain to retain more.
This might work best with non-fiction books, although you could also prep your brain for remembering fiction too (especially long novels with tons of details like Game of Thrones).
Check out Battles' post for many great tips on speed reading as well.
How to Become a Faster Reader [Ryan Battles]