From the Inside Flap:
The last thing the girls wanted that summer was William. Living miles from town and their friends was bad enough. Now with a twelve-year-old cousin to entertain, they were sure the vacation would be ruined.

But that was before they met William, and before he found the mystery right in front of their noses.

Conrad Barrington, a rich neighbor, planned to build a housing development on Cinnamon Hill. Everybody hated to see the Hill destroyed, but no one had dreamed of tyring to save it - no one, that is, except William. He had an invention, he told them loftily, which could do just about anything, and he and his machine would stop the project. The only problem was that William uncovered several puzzling facts about Cinnamon Hill, and his bold detective work brought real trouble to the family.

Perhaps William's invention solved a mystery, and perhaps it didn't. By the end of the summer, one thing was certain, though. Nobody in the family - in fact, the entire town - would ever forget its remarkable owner.

(ex-library copy, rescued from the recycling depot)

Into the new Little Free Library.