The Smoke Ring

The sequel to the Integral Trees (which I'm sure is around here somewhere).  The Smoke Ring by Larry Niven is one of my favorite scifi settings of all time.

Imagine, if you will, a gas giant.  Orbiting this planet is a torus of gasses, that just happen to be the right composition to support Earth-like life.  Plant and animal life flourish in this free fall environment.  Spherical islands of vegetation dot the sky.  Colossal treesso large that the winds at either end are moving in opposite directions giving them a distinctive shape similar to the mathematical symbol for 'integral'.  A ship full of humans find themselves stranded in this place.  The action in these two books takes place many, many generations later and a society unique to this unique environment has evolved.

This cover really expresses the feel of the environment.  Since everything is in constant free fall it is possible to 'fly'.  It is also possible to drift away from anything solid or be picked off by predators who actually evolved to live here.