The Tower at the End of the World — John Bellairs

When Lewis, his uncle Jonathan, and their friends Rose Rita and Mrs Zimmermann take a trip to a small town near Lake Superior, they expect a pleasant holiday. Instead, they find themselves facing the ghastly Ishmael Izard, son of the fiendish creator of the Doomsday clock that was once hidden in the walls of Uncle Jonathan's house. Ishmael is a cruel and heartless sorcerer, and he is determined to wreak vengeance on the entire world.

Will Lewis and his friends be strong enough to defeat him, or will their fate be decided by their most formidable foe yet?

For fans of Goosebumps.