Almost forgot...
Dunno if this falls within the scope of your plugin but as an additional feature for the book id's if it would be possible to use customised ids to reference fanfic sites e.g.
http://www.fanfiction.net/s/5149431/...han_Fanfiction
would be something like:
ffn:5149431
or
http://ficwad.com/story/181839
would be fwad:181839
Hopefully you get the idea I'm tryin to put accross