And The Devil Will Drag You Under (Jack Chalker 1979) is about an alcoholic demon's quest to save the world from collision with a large heavy object (in this case an asteroid.) The demon contrives to send two humans on missions to collect the magical orbs needed to persuade the large heavy object from continuing on its collision course.
The missions are to different worlds with different rules and physics to our own, but each world has a reality and depth of its own. Chalker was skilled at world-building and each mission is memorable. The book is episodic, the two humans only combine forces at the end of the book. Common Chalker themes like transformation of the human body, magical science and scientific magic, and variation in the laws of physics, come into play.
The book's tone is often humorous but there are important ethical choices - how far will the humans go to save our world? What will they compromise? And there aren't always nice answers.
Chalker has a very satisfying ending to the book (not all of his books or series have great endings but this one does have.) This is a book I read every so often, not just once or twice.