I think the only book I made an effort to hide was one my friend let me borrow like 6 or 7 years ago (I still have it, because I haven't seen her in years). It was about lesbians, so I didn't want them to get any ideas (if they didn't already have them) There wasn't really much (if any) sex in it either. The book was ok, but I wouldn't call it a "favorite." Most of my favorites were (and are) written for kids and/or teens anyway, since that's what I like to read most of the time.
Although I think if my dad had actually read all the books in the "Clan of the Cave Bear" series by Jean Auel, he probably wouldn't have continually checked them out at the library for me, and then I would have had to check them out myself and hide them (my parents rarely paid attention to my library books - I mean, I read a couple trashy romance novels when I was 9, for crying out loud...). He had read part of the first book, but must have abandoned it before all the rape and violence and stuff. He had no idea how much graphic erotica there was in the rest of the books, though. I learned so much about sex from those books, and I read them all when I was only 13 (except the last book, which was published about 5 years later, and I read about half of it and intend to pick it up again at some point). Of course, I read it more for the story than the sex (suuuuuuure, Ade...), but my dad had no idea what he was encouraging me to read. Bwahahahahaaaaa.
Also, Judy Blume rocks my socks off. Never had to hide those books. I displayed them proudly.