Title: Midnight at Well Souls
Description: From the back cover:

Who was Nathan Brazil... and what was he doing on the Well World?

Entered by a thousand unsuspected gateways -- built by a race lost in the clouds of time -- the planet its dwellers called the Well World turned beings of every kind into something else.  There spacefarer Nathan Brazil found himself companion ed by a batman, an amorous female centaur and a mermaid -- all once as human as he.

Yet Nathan Brazil's metamorphosis was more terrifying than any of those... and his memory was coming back, bringing with it the secret of the Well World.

For at the heart of the bizarre planet lay the goal of every being that had ever lived -- and Nathan Brazil and his comrades were... lucky?... enough to find it!
Subjects: Science fiction