What kind of books? I discover that I'm usually cherry picking chapters or section in technical or books where there is no strong interrelationship between the various elements or the topics. As already pointed previously, marking or taking notes inside those kind of books is useful to keep a trace and easy find back what you discovered or learned in a technical book.

There is a good reading about taking notes and especially the approach of thinking while relying on our current readings : Notebooks of the Mind, Exploration of Thinking, Vera John-Steiner (ISBN13 978-0195108965).

We are often learning at school to always finish a book even if we don't like it or we have some difficulties to read it. I tend to agree for the difficult books where sometime, you have to force yourself to finish it. But sometime, giving up is not so bad and relaxing your mind with more "easy to read" or "easy flowing" books is helping us to relax before reading more complex books. That's why I always keep around 4 or 5 books in my reading queue... and jumping back to some of them when my mind is more ready.