The Great Brain - John D. Fitzgerald
This first book in the series is a great combination of The Adventures of Tom Sawyer and The Terrible Two series, and is perfect for fans of Roald Dahl.
The best con man in the Midwest is only ten years old. Tom, a.k.a., the Great Brain, is a silver-tongued genius with a knack for turning a profit. When the Jenkins boys get lost in Skeleton Cave, the Great Brain saves the day. Whether it's saving the kids at school, or helping out Peg-leg Andy, or Basil, the new kid at school, the Great Brain always manages to come out on top-and line his pockets in the process.