The House With a Clock in its Walls by John Bellairs

VINTAGE PAPERBACK [1980] DELL YEARLING

Excellent copy with some shelf wear to cover, mostly along edges, and tanning to pages. 

Illustrated by Edward Gorey.   Orphaned Lewis Barnavelt comes to live with his Uncle Jonathan and quickly learns that both his uncle and his next-door neighbor are witches on a quest to discover the terrifying clock ticking within the walls of Jonathan's house. Can the three of them save the world from certain destruction?