I’m gonna go with The Unbecoming of Mara Dyer by Michelle Hodkin and Can You Keep a Secret? by Sophie Kinsella. The Unbecoming of Mara Dyer is so mysterious that I think it would appeal to a fairly large audience and Can You Keep a Secret is funny as hell!
Side note: I think One for the Money would have made a good TV series. There is no way people are going to stay interested enough for 18 movies.