99c, ON SALE FOR CYBERWEEK, FROM ZOEY IVERS:   The Barton Street Gym (Doors into the Dimensions Book 1).

In a future where no one needs to sleep, homes are unnecessary. A simple dimensional cubby in a hi-rise gymnasium is all anyone needs.
Perhaps the designers ought to have considered the possibility of unintended effects from thousands of multi-dimensional bubbles in close proximity to each other. But they weren’t physicists . . .

Joe is independent, his divorced father a bit absent minded. Joe and his “bio-model” friend were exploring the pedestrian tunnels under the Barton Street Gym when they found themselves trapping in a strange landscape with a hungry dinosaur.

Alice Brown is used to getting in trouble; her parents are quite strict. But being attacked by a Tyrannosaurus Rex in a strange dimension just a step across a malfunctioning D-door is not her usual sort of trouble.