I was looking through all my books the other day, and I realized that I could neatly separate them out into three categories. Normally generalizations like this don't work, but without a doubt every book I owned fell neatly into one of these three categories:

Books that are read for content:
    These are the dusty books in my bookshelf. They are the unloved books, the ones that were read once. Often for a class, they were quickly skimmed through so that I could obtain maximal understanding for minimal time wasted reading them. I feel almost bad for these books, all of which sit there neglected, and unread. Often they quickly will find their way from there to the Used Bookstore without much effort or guilt on my part.
Books that are read for fun:
    The books I've read for fun remind me of things I read as a kid, staring wide-eyed and naively into the pages of old books, absorbing magically the adventures that went on. I haven't entirely lost that sense of excitement and suspension of disbelief. The books that fit under the fun category are all very dogeared. Not a single one without a cracked spine. It's not that I don't take care of my books, but more that after five or six readings it's very difficult to keep them pristine. All of these are fiction, but range from SF to even historical fiction.
Books that are read to think:
    Something about a way a book strikes me, usually in the balance of excitement to thought, often causes me to read with a pen. I get excited about these kinds of books, ofttimes philosophy, in which I create my own ongoing argument and dialog with the author of the book. I underline, I number, I reference. I love going back and reading these books again and again, because I see my thoughts, my little doodles on the side of the page that says "This author is on crack! He contradicts himself right here!" and "WTF is this tangent for?". They're not always the most intelligent comments, but I still write them in anyway because it causes me to think about what's going on, and to get a lot more out of a book.

    A lot of people think that the reading with a pen can be destructive to the poor book that you're marking up. For me, writing my thoughts in a book is a sign of love, that I care enough about this book to put a little bit of myself into it, and to take the time to think about every little thing it's saying.