Annabeth is okay. I like it more than
Annabel. It seems less strongly associated with the character than
Hermione is with
Harry Potter, so it would make a nice reference to a book without seeming too out there.
Elizabeth >
Mary Beth >
Annabeth >
Beth >
Bethany >
Bethan > Mary-Beth >
Marybeth
I've never heard
Beth pronounced like
Biff.
Biff sounds like the bully in "Back to the Future."