I personally forget the majority of what I read and only remember a few main ideas. With technical books I take notes in google docs and try to review them later to cement the ideas in my mind.
That's a really good question, and one I ask myself a lot. I honestly don't know, and I've read reference to some studies somewhere or other that suggest that most people retain fairly little of what they read. So my strategy with a lot of books is to read the book, not take notes or anything really, and at the end, make a simple "this is worth revisiting" or "this isn't worth revisiting" decision. If it is, I'll read it again and either do some light pencil underlining or use post-it page flags to highlight important stuff, OR I'll do a full-fledged "create a wiki page for this book on my wiki and take detailed notes" pass.
Sometimes I'll do the "light pencil underlining and post-it notes" thing on the first pass.
It's an interesting trade-off... I don't want to spend too much time taking notes and cut into my time for reading other books. But I don't want to waste the time I spent reading a book and not retain anything from the experience either!