Here's a post from Miss Snark's now-discontinued blog, with comments about what people are least likely to want to read about. The consensus reached: nobody will read a book where a child gets raped.
I've attended convention panels that came to the same conclusion, one panel member after another insisting that the topic of child rape is definitively uncommercial. Which naturally begs the question -- why did everyone read Khaled Hosseini's The Kite Runner? If you make the rape of a boy a poignant metaphor for the fate of Afghanistan, and if your book comes out at a time when people suddenly, if belatedly, care about Afghanistan, maybe your book will be a phenomenal international best-seller.
Nobody has a clue what will sell -- see tomorrow's post. By the way, if you post a comment and it doesn't appear on the site, please e-mail me about the problem: we're experiencing some kind of technical problem with the comments that we're still trying to sort out.