Summary of the boxes by William Sleator?

When Annie's Uncle Marco is going on a trip, he leaves Annie with a responsibility: to take care of two mysterious boxes, one made of wood, one made of metal. He tells Annie that the boxes must be kept separate and no one could know about them. They cannot be opened at all. Naturally, Annie is consumed with curiOSity, and after a while, opening the boxes is all she can think about. Eventually she pries open the wooden box, which she has stowed in the root cellar of the big old Victorian house where she lives with her fat, mean Aunt Ruth. A tiny crablike creature comes scuttling out, and disappears into the darkness of the basement. The metal box that is hidden in Annie's closet, turns out to contain a strange clocklike device, marked with odd hieroglyphics, or graphical figures.Somehow, opening the second box activates the clock, which now seems to be measuring time, though not in any way Annie can understand. Meanwhile, in the basement, the crablike creature has begun to reproduce. The resulting tribe of beings is able to communicate telepathically; they inform Annie that by opening the boxes she has become the "nervous system," a link of communication between them and the clock, which they appear to worship as a kind of god.

Annie and her friend Henry, in whom she has confided, discover that the clock, if asked in a certain way (and provided with sacrifices) can actually slow time. But the evil Crutchley Development, a construction company that wants to tear down the old neighborhood where Annie lives and build a huge mall in its place, has hatched a plot to steal the clock and use its time-slowing power for their own greedy ends. Annie and Henry must find a way to prevent this from happening, foiling the company and saving the neighborhood.