The Secret of the Underground Room — John Bellairs

A young hero and his professor friend set out to save a priest from a ghost, in this novel by the author of The House with a Clock in Its Walls
 
Aside from the eccentric Professor Childermass, young sleuth Johnny Dixon’s best friend may be Father Higgins, the kindly priest at the local church. When Higgins is transferred to the congregation in the tiny town of Rocks Village, Johnny and the professor are afraid they won’t see their old friend ever again. But they’ll be reuniting with Father Higgins sooner than they think—and the thing that brings them together will be positively out of this world.
 
No sooner has Father Higgins moved than he begins seeing a ghost lurking around the church. The apparition is a young girl who never speaks, but has a habit of leaving cryptic notes around Higgins’s house. When Higgins disappears, Johnny and the professor follow his trail, embarking on a haunting quest that will lead them all the way to England.
 
In The Secret of the Underground Room, this multimillion-selling, Edgar Award–winning author offers a good old-fashioned ghost story packed with adventure and suspense.