I strongly prefer checking books out of the library to buying them, because I prefer pretty much any way I can get a similar result without spending money. The downside, to some, is that library books have been read by many other people, who tend to leave their tracks. I find this part of the fun.
Most commonly one finds the printout of someone else's library receipt being used as an impromptu bookmark. It's like a ready-made recommendation list based on your reading choices. Other bookmarks are also intriguing: ticket stubs, junk mail. In one of my recent reads, I found a (wrapped) stick of gum tucked into the card pocket. Very handy.
Food stains are disgusting, of course, but understandable. Eating and reading belong together. The Christmas I had homemade chocolate truffles to eat while reading War and Peace (a great story in between the essays) still shines bright in memory, though I wonder if I could have gotten through it without the truffles.
Then there's the slightly wrinkled pages, which in my experience come from combing wet hair while reading, but perhaps there are other ways to get there.
Marginal notes are not as disgusting, but more indefensible. It's a library book. You don't write in it. Other patrons do not want to know your answers to the quizzes, and your thoughts are probably not brilliant enough to enhance the book.
What's the weirdest thing you ever found in a library or used book?
EDIT: Wondergirl just reminded me that I forgot to mention the Band-Aid I found in a book last week--probably the thing that originally prompted this post. It was, fortunately, wrapped and unused. Handy for paper cuts.