de Camp uses his common fantasy setting of Novaria for this book, and throws a typical plot twist by having a demon as the central character. Mind you, a demon in this sense is simply a being from another dimension, and Zdim happens to be a mild mannered, scholarly sort of being who (for instance) tries to trade off on his boyhood freindship with the magistrate to avoid the summons to serve on the human plane in exchange for iron. Zdim just wants to stay home, bite his wife, raise rabbages and help hatch the kidlings.
But he is denied a wavier and whence the adventure begins.
de Camp's one central grace for me is he writes about people. His villians will look at you and say, "Me, a villian? But you, dear sir, are far more a villian!" And they mean it, spouting viewpoints which are (in the villian's sense) perfectly logical (if not exactly moral). Culture clash is often the center of his stories. Take a demon skilled in logic and reason and throw him in with barbaric humans and you wind up with non stop exasperation and amazement at the duplicity involved. As Zdim points out, 'feindishly clever' is quiet a strange racial tag for the incessantly cunning and devious humans to come up with!
"I endeavor to give satisfaction," is perhaps the exasperated catch phrase of the de Camp books.