First in the Well World series. A spaceship carrying grain and a motley assortment of travelers (a factory cloned messenger, a drug merchant and his addict slave, and an old space captain shrouded in mystery) stumble across the remains of a group of graduate students that had been studying an ancient civilization's ruins. They follow the remaining members of the group into a transportation well that leads to the Well World, a remnant of the ancient civilization that still functions...
This is the first book in the Well World series. Nathan Brazil finds himself on a bizarre world, which can be entered though any number of portals. There are a couple of problems: first, it is much easier to go through the portal than it is to come back, and second, using the portals causes some very strange changes in one's biological makeup.
While on a bizarre planet where beings of every kind undergo strange metamorphoses, spacefarer Nathan Brazil finds himself companioned by a batman, an amorous female centaur, and a mermaid.