Hi there,
Wonderful hack this one, my users are loving it.
I have found a way to add a book to an existing thread by forcing a URL into your Browser.. not sure if someone's done this before or if there are any ill effects of doing this, but I thought I'd share it.
I just looked at the URL you are re-directed to after adding a thread with a vbookie event, simply change the threadid. In my case I change the red bit.
http://www.gothetahs.com/forum/vbookie.php?t=10699&vbookieeventoptions=0&do=newevent
Cheers,
Moses.