The House with a Clock in Its Walls by John Bellairs
Lewis Barnavelt is a chubby middle schooler whose parents recently died in a car accident. He has just arrived in a new town at the house (mansion, actually) of an uncle he hardly knows. Uncle Jonathan is eccentric, as is his neighbor and best friend, Mrs. Zimmerman, a middle-aged widow who loves the color purple.
As Lewis begins to adjust to a new living situation, new school, and new neighborhood kids, he gradually becomes aware that there’s something weird about Uncle Jonathan and his house.