Midnight Magic, by Avi, 1999

Fabrizio, 12-yr old servant to ex-magician Mangus, accompanies his master when summoned to the castello to help the King by exorcising a ghost who haunts the princess Teresina. Mangus, who has sworn off magic trickery, believes in reason, not ghosts, and he despairs of his life, for if he does not find the ghost, King Claudio will kill him, and the king's evil advisor, Count Scarazoni, will kill him if he does. There's nothing left but for Fabrizio to take the mystery into his own hands. But is he a player or a pawn in a ghostly game of chess? Strongly recommended for fans of historical whodunnits.