The House With a Clock in Its Walls — John Bellairs

Lewis had always wanted to live in a house like Uncle Jonathan's -- full of marble fireplaces and secret passageways and dozens of unused, unexplored rooms. And living with Uncle Jonathan, a real wizard, was full of fun and surprises.

But while Uncle Jonathan practiced funny and comfortable white magic, the original owner of the old house, Isaac Izard, had been an evil sorcerer. He had devised a plan for bringing about the end of the world. Somewhere in the walls of the house he had hidden a clock. Every night Lewis and Uncle Jonathan could hear it ticking -- sometimes loud, sometimes soft -- marking off the minutes until doomsday.