I read this book at the Boulder Library, it was written in this century (I think).
Earth humans are subjugated by a technologically superior species. At first the foolish Earthlings (are there any other kind?) think the superior species is a bunch of evil meanies. They come to find out that the meanies are actually helpful in protecting humans from far more pitiless and powerful adversaries, who have factions called Harmony and Melody, and when a defector from the adversary conveys information to the meanies, it is compelled to destroy itself because it’s contaminated by inferiors.