Definitely Maybe by Arkady and Boris Strugatsky

First thing you need to know: this book has no relation to the movie of a similar name that starred Ryan Reynolds and Rachel Weisz. That movie has a Comma in-between the two titular words, and is about something completely different.

This is a perfect little book. 140 or so short pages, written by the Strugatsky brothers in 1974. Fun from beginning to end, great beginning, excellent ending, and introduces (to me, at least) the concept of a Homeostatic Universe. I won't say anything more about its plot.

You should read it. Tragically, like all of the books in this Soviet Sci-Fi series, it is out of print. If you want to read it but don't feel like ordering or finding a used copy, let me know and I will happily loan it.