Adding "Science Fiction" as the genre for the book links the book in the "Science Fiction" topic both under "Books In This Genre" (which sounds right) and "Books About This Topic", which sounds wrong. Have a look at the "Snow Crash" entry.
http://www.freebase.com/view?id=%239202a8c04000641f8000000000035120
Discussions on Book Subject
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Actually, this happened because Science Fiction was already assigned by somebody else as a Subject of Snowcrash, which it shouldn't have been.
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The subject/genre confusion happened in part because "genre" was added as a property of "book" some time after "subject", so subject was doing double-duty for awhile. Note, however, that "science fiction" is a perfectly legitimate book subject (although not for "Snow Crash"), since there have been any number of books written about the genre itself.
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