I'm not a fast reader, sadly. But I tend to get the most out of books when I make an effort to "be alone" with the book. Sounds weird, but it's more like you're having a discussion with the author. And so you want to give the book your undivided attention.
Only till recently have I started writing in my books. I find that after all these years I've missed out on an important aspect of reading. Writing or underlining is sorta like you talking back to the author in the conversation.
I also like reading what other people have written or underlined in books. To see what went through their minds or what was important to them.
And to get the most quality out of the books I try to dwell on what I'm reading, with hopes that I would remember it in the future... and that my life would be better by it.