The Secret Cellar — Michael D. Beil

Mysteries seem to find the Red Blazer Girls: when Sophie finds a secret message in the antique fountain pen she bought for her father, the girls are on a case. Soon, they're at the home of the pen's original owner, a secretive man who kept to himself. His house is full of puzzles, all of which protect a hidden treasure, and the Blazers will do anything to get to the bottom of things. Throw in an ill-tempered bookstore owner with a motive, a missing will, a walking stick that doubles as a key, a school Christmas play, and a rat named Humphrey, and it all adds up to another thrilling adventure for the Red Blazer Girls.