Human Filtering... a woman checks out multiple copies of a sex education book from the library and refuses to return them, proudly proclaiming that she is taking them to prevent any children from reading the books. As a result, the library has to order more copies of the books due to requests from the publicity resulting, and the book gets more sales.
Vigilante censorship doesn't work.
Out of curiosity, I went to my local library and looked at the book in question, "It's Perfectly Normal." I flipped through it and immediately saw what the book thief was no doubt horrified by: accurate cartoons of the human body showing sexual organs. I read through the words that accompanied the offending pictures, and found that it was a very straightforward, basic text on the human body. The kind of book I really deeply wish I'd had when I was a teenager/pre-teen going through puberty. The book emphasized acceptance, intelligent decision making, and hygiene. Altogether, a really great book that I'd be happy to get for my own kids (if I had any).
Of course, it's also a book that requires a parent to actually be involved in parenting. You don't just hand this to a kid and say, "here, figure it out!" Clearly, though, parenting is too difficult for parents like the book thief. Better to just make sure the kids never see anything you disagree with. It's like The Man That Corrupted Hadleyburg all over again.