I used to be frighteningly careful with books. When I was a kid, of course, I drew in them, ripped pages and covers, and generally mucked them up, but by the time I was in university I kept all my books in perfect shape. Never creased the spine. Never dog-eared pages. Never even used a highlighter in biology textbooks that would nevertheless be out of date in a year or two and practically unsellable -- not that I ever sold them anyway. (My wife and I, who both have biology degrees, now have a lot of duplicate textbooks on our shelves. We almost never look at any of them.) Most pathologically, I even avoided writing in the phone book.
Then, in 1992, when I was working on my writing diploma, I bought one of Thomas Kuhn's books on scientific revolutions. I made a conscious decision to make a mess of it. I used a highlighter, made notes on the pages, folded corners -- essentially treated it as disposable.
I still have the book, of course. But in the last ten years I've found a happy middle ground. I treat books casually, not intentionally doing any damage, but not worrying about them either. Sometimes the spines crack, sometimes I'll fold a corner on a particularly interesting page, but most of the time my books remain in pretty good shape. But I do circle numbers in the phone book now -- if I remember to force myself.
P.S. As my family and friends will tell you, I am only selectively neat-freakish, mostly in places where it's not helpful. Sure, I'll obsess over a book or the folder structure on a hard drive, but laundry piles up in my big basket for weeks, and I never remember to water the plants.