The Mysterious Benedict Society and the Prisoner's Dilemma
Trenton Lee Stewart
PB ISBN 1906427690
These books are a wonderful contemporary take on the genre dominated by Famous Five, and so much more interesting than most children's mysteries. The clues are there for your child to figure out, and the plots and character development don't belittle the young reader. The villains are well-gauged: genuinely evil and frightening, but not the stuff of nightmares. Perfect for 8-12 year olds.