Nowadays I use spaced repetition software to capture anything interesting I see in a book (including insights that occur to me while reading), so I basically remember everything I want to. Before using SRS I might remember ~20% of a book.
I use ThoughtSaver (www.thoughtsaver.com, currently in very early beta), but most other people I know who use Spaced Repetition prefer Anki (http://ankisrs.net/). Gwern has a good introduction to the general concept of Spaced Reptition: http://www.gwern.net/Spaced%20repetition