All life on Earth will die of thirst unless a couple of loners on the run can use a strange time machine to stop a secret project! An action-packed short novel from a Nebula Award nominee.

When Meg's mother died in a hospital mishap, that set her father, a scientist—now an angry scientist—on an obsessed journey to develop a tiny time machine (TTM) to save her. Before he could perfect his new device, however, he died in a confrontation with his business partner.
Enter Meg, the angry scientist's daughter (as opposed to a mad scientist's daughter). She's a loner, on the run with another loner, Josh, when she finds her dying father and the TTM.
It's all up to Meg and Josh to fix a disaster set to render the Earth uninhabitable, dive back into the past to rescue Mom and Dad, and overcome an adversary who wants the TTM for his own purposes.
Complete in this one volume:
Tiny Time Machine 1: When Meg Met Josh
Tiny Time Machine 2: Return of the Father
Tiny Time Machine 3: Mother of Invention
About Stith's prior work:

"Stith writes in the best hard-sf manner, dropping characters into a situation that can be solved only by thought and reason, but he also, more modernly, creates real and believable characters. He is becoming one of the most eloquent modern hard-sf practitioners." — Booklist