Whatever your age I don't think you can go wrong with a Harry Potter, they really are a great read, and you can get versions with grown-up covers, so you don't look too grim. If they're not your cup of tea a collection of Roald Dahl short stories (for adults) are good, as macarbe as his children's books but all about sex, drugs and murder. If you want comedy with a difference get Bill Bryson's "Notes from a Small Island", which is hilarious; or if you want to lose yourself in history, comedy, romance and tragedy then I can't recommend "Captain Corelli's Mandolin" highly enough - I'd suggest starting at chapter 3, because the first 2 are hard work and not really relevant to the plot.