The Sorceress's Mask

When Violet the Librarian finds out someone is borrowing books without a library card she starts to investigate.
It turns out the person borrowing the books illegally is actually an elf.
An elf that loves books as much as she does.
An elf that gets kidnapped.
An elf that needs her help.
This leads to high fantasy adventure and the an attempted rescue.
What is the secret behind the Sorceress's Masks?