YAAY!!! Lots of Agatha Christie readers....(Beau, Then There Were None IS kinda scary, but that's why I love it!) When I was 12, I went from reading Nancy Drew to Agatha Christie, and I still enjoy picking up one of her books now & then and reading them!
I'd like to go ahead & reccomend Isaac Asimov, particularly I, Robot. You do't have to be a sci-fi fan to find it enjoyable, but it may not hurt! Robot Dreams, his short story collection, is good as well.