I've heard that this mystery is popular, though it doesn't seem to go out that well at our library (I don't think the cover on the hardback is very appealing). It seems like it'd be most popular with 5th to 7th-grade girls. Set primarily at a private Catholic school in Manhattan, it has an accessible, fluffy writing style, as if your 7th-grade best friend is talking to you. The main characters are smart, and it introducing some math and literature concepts as part of the mystery. A little romance in there too, pretty standard stuff.