Man, you bloomin well aced this!
It's working fantastically on my website already (must implement some security - can u say 'webrabbit'?)
Just 1 comment. You say 'books by category' where you've actually used 'tags' and category is a different concept within stanza. Perhaps rename category to tag, and then rename series to category (feels like a more direct mapping)
PS: You're code is cool; looks like a lot of work went into this.