I've been geeking out with organizing my books on Calibre, so for what's it's worth here is my system: Every book gets the tag of fiction or nonfiction. From there it usually get at least one or more other tags like Modern Lit, Classic lit, Sociology, History, Biography, Thriller, Mystery, Science Fiction, Fantasy, Historical Fiction, etc. And then if it was written by a non-US/UK/Canadian author, I might throw in a tag like Japan, Scandinavia, etc.
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