my younger sister (17) and I are both really enjoying a kidlit series we started years back when she was actually a kid. They fall into the kidspies! genre, but seem to mostly give the kids realistic personalities and limitations which is where the other books of the type seem to always go wrong. Anyway (rambling a bit there), they're the Cherub series by Robert Muchamore, I think the first is called The Recruit. Plus, they aren't too long, so you won't waste a tonne of time if you hate them (they aren't the best written books, but they are fun and interesting).