If you've never read anything in the unusual genre of science fantasy that blends science fiction and magic, this is the place to start. It's one of the classics of the unique genre.
This book consists of four connected novellas published between 1956 and 1969 in The Magazine of Fantasy & Science Fiction: "Operation Afreet," "Operation Salamander," "Operation Incubus," and the three-part "Operation Changeling." Anderson added linking material for book publication in the 70s.
Heroes Steve (a werewolf) and Virginia (his witch wife) fight against a demon being used as a superweapon in World War II, stop an elemental college prank gone amok, confront a succubus/incubus on their romantic getaway, and enter the hell dimension to save their daughter. The tone changes between the different segments: the college story is riotously funny and played almost strictly for laughs, while the lengthy final novella emphasizes heavy science, a deadly-serious quest, and thought-provoking satire on religious zeal gone wrong. But ultimately, I'm not complaining: this is top-notch science-fantasy and an example of what an incredible talen the late Poul Anderson was. His logical approach to fantasy makes magic and the supernatural into scientific forces that operate in his fictional universe the same way that modern technology operates in ours. The world of _Operation Chaos_ is recognizable as 20th-century America, except that magic is this world's science, and is treated in the same way that scientific theories and inventions are in our own. Anderson handles this difficult conceit to near perfection, writing fantasy with the techniques of science fiction.
Come on, take the ride into chaos!