If you go to the American Library Association site you will find a list of banned books by year. The list states why the book was banned or challenged. I want to list some of the books from the list which I have read. These are from the 2011 list: Diary of Anne Frank, Water for Elephants, Slaughterhouse-Five. From 2010 list: I Know Why the Caged Bird Sings, The Bean Trees, To Kill a Mockingbird. And from 2009: Midnight in the Garden of Good and Evil, The Kite Runner, Wicked, Huckleberry Finn, The Lovely Bones, The Color Purple.
All of the above books were interesting and challenging and well worth the read. One of the books that is on the list is A People's History of the United States by Howard Zinn. I have had it for years and not read much of it. I think I'll get it off the shelf and read more!