Gravity Wells and Flower Clocks: William Sleator, Marco's Millions and The Boxes
Marco's Millions. New York: Dutton Children's Books, Penguin, 2001.
The Boxes. New York: Duttons, Penguin, 1998.
(NB: these dates can't be correct as Marco's Millions comes first, but they are what I seem to have),
A science fantasy around time mathematics, but nothing terribly complicated.
Marco's sister is small, slight and frail, When she finds a time tunnel in the basement and dreams beckon her through, she sends her brave brother Marco instead, Marco is fascinated by distances and he discovers another world on the other side where bugs want him to travel on a mission for him to stop a black hole from collapsing. Although Marco's first three trips tell him that time runs faster on the other side, he doesn't get his calculations right and when he comes back after three weeks he discovers that the further in he went, the faster time has sped by. When he gets home his parents are dead, his sister is grown up but died in an accident leaving a small baby--Annie--and his youngest sister, always a pain in the neck, has become very unpleasant. He leaves on his travels and promises to check in on Annie.
Annie has grown up with Ruth, Marco's younger sister. Ruth is a stereoptypical lazy slob. Marco leaves Ruth two boxes and tells her not to open them. She does and one of the little bugs from Marco's Milions comes out The other turns out to hod a time clock--an organic mechanism. Annie and her friend Henry get caught up in the bugs' plans, in slow downs and speed ups, and that in turn becomes the object of desire of a construction company tryng to buy up the area.
The clock gets stolen, Annie and Henry get the bugs to ask for a slow down and get it back and Marco explains all. It starts well but this is not as good as most of Sleator's books: the science of time and gravity barely gets a look in and the explanation at the end is classic "I will now tell you who the murderer is."