Well.....some of the topics covered in the book are self-torture, murder, and ritualized sex. However, all are done in a very non-involved manner (as in, there are no steamy sex scenes or gratuitous gore). Also, as you're no doubt aware, the book suggests things about certain religious/historical personages which some people find disturbing or offensive. I know one person who burned her copy after reading it.
I would totally let a 12 year old read it, but then I don't believe in censoring what kids read.