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

Lewis Barnavelt is back and this time he has stumbled upon something more powerful than even his 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 safe? Is Lewis strong enough to face the potent spirit that he is unknowingly summoning?