I'm glad someone here mentioned Courtship Rite. I wouldn't exactly characterize it as being about cannibalism, although that's one aspect of the book that people seem to ping on strongly. At least there's a good reason for the cannibalism: nth-generation colonists are stranded on a really shitty world where there's no indigenous fauna except insects and most of the flora are seriously poisonous. So the only sources of protein available are bugs and other people. The group marriage featured in the book and the politics of the whole thing (generally creepy and brutal) are interesting, though.