[quantombone]

I use a pen when I read. I underline important bits and actually write down the key points in the margin. Writing these things down helps memory and I don't really care about leaving the pages in the same state that I found them. Also I don't always have a notebook with me, and I do this to my own books only. I just don't care about "ruining" a book -- I want the knowledge!

Of course you might not want to borrow some of my books when I leave them in this state -- they look like study aids from a med school study-a-thon. But whatever trick YOU find that helps you pound knowledge into your brain is worth it.