The Whistle, the Grave and the Ghost - John Bellairs

A camping trip with his Scout troop leads Lewis straight to a strange, old grave and a mysterious, magical silver whistle. When Lewis discovers that this whistle has the power to stop the boys in his Scout troop bullying him, he is overcome with relief—finally he can enjoy being part of the Scouts. What Lewis doesn't know is that the whistle also has a more sinister power: it summons a powerful vampire! Lewis is drawn into battle with this ancient evil, and with the help of Rose Rita, Uncle Jonathan, and Mrs Zimmermann, he must save his own life and the lives of those he loves. Is Lewis strong enough to destroy the potent spirit? For fans of Goosebumps.