Midnight at the Well of Souls begins Jack Chalker's Well World series. Tacking WORLD onto a concept was the freshest meme back then, and the Well World certainly delivers. When I picked up the book, I didn't expect much, but I found the prose style energetic, up-paced, and entertaining. This was a much better read than I was expecting. And like most of the SF of that era, it wraps itself up in one book.
The story itself is a sci-fantasy. That's all due to the concept of the Well World, where many different races live in hexagons on the planet's surface. They were all artificially created by a race called the Markovians, and the planet itself is run by a world brain. Thus, you have super-high tech appearing as magic, and many otherwise fantasy creatures, such as centaurs, mermaids, and hyper-intelligent concepts. (Really).
Yet, that's not the story. That's just the concept.
In the story itself, the passengers on a freighter, responding to a distress call, find themselves dragged into the Well World, given new bodies, and begin a race to reach the control center of the planet. To do that, each group must lie, cheat, steal, and cross alien and hostile hexes in order to get there first.
The protagonist is a freighter captain, and inhumanly old Nathan Brazil, who doesn't much like what the human race has come to. Identical service clones are not his idea of a good time. When pulled into Well World, it soon becomes clear that there's more to Nathan than meets the eye, and he knows more about the Well World than he's letting on.
The book has a little sex, but not graphic enough to bother with. If you're easily offended by inter-species sex, and all the possible variations of offended implied by changing bodies, which also means changing genders, then this might not be a good book for you. Even so, the risque is mild by today's standards, and I don't think that most folks would notice much.