A growing number of book lovers are using tags to provide their own way of classifying books. Amazon and some library catalogs have introduced user-generated tags to supplement hierarchical systems, like Library of Congress subject headings.
Book tagging enables anyone to assign trendy, granular labels to books with more authority than a librarian. For example, there's no library category or Amazon tab for steampunk, a subgenre of speculative fiction. But using tags, aficionados can dissect steampunk into all its sub-subgenres, including timepunk, bronzepunk, stonepunk and clockpunk--all very different animals to steampunkers.
Likewise, at least a half-dozen subgenres are within what many people call "queer fiction." But you won't find subject headings for any of it in a library. Instead, these books are shelved in "City Life" or "San Francisco," which doesn't help anyone find them. Traditional subject headings don't connect with personal identity, but tags can.