The Whistle, the Grave, and the Ghost (Lewis Barnavelt #10)2003

Lewis Barnavelt is back! And this time, our lovable underdog/hero has stumbled upon something more powerful than even his overactive imagination can dream up. A camping trip with his Scout troop leads Lewis straight to an old grave and a mysterious, magical whistle. When Lewis discovers that this whistle has the power to stop the boys who insist upon bullying him, he is left with a tough decision. Does he continue to use the whistle, which he suspects might not be fully benign? Soon the magical whistle draws him, his best friend Rose Rita Pottinger, his uncle Jonathan, and their friend Mrs. Zimmermann into a battle with an ancient evil.