House with a Clock in its Walls, The (Vintage Paperback) by John Bellairs

When Lewis Barnavelt, an orphan, comes to stay with his uncle Jonathan, he expects to meet an ordinary person. But he is wrong. Uncle Jonathan and his next-door neighbor, Mrs. Zimmermann, are both magicians! Lewis is thrilled. At first, watching magic is enough. Then Lewis experiments with magic himself and unknowingly resurrects the former owner of the house: a woman named Selenna Izard. It seems that Selenna and her husband built a timepiece into the walls—a clock that could obliterate humankind. And only the Barnavelts can stop it!