w00t? A billion attributes on hundreds of billions of rows? Could you think more of an edge case? You're just coming up with shit so you can show off how l33t you are. If you have that kind of a need, just go for a NoSQL server, nobody is stopping you. This feature is a nicety to have, not a replacement of NoSQL.
You just need to create a new table which links tags to books. That's how relational databases are normalized ...