Wild Magic by Cat Weatherill is $1.99 for ebook format as part of Amazon's The Big Deal: Kindle Books Up to 85% Off through 11/23/14.
This book is the most recent in the genre of retellings of the Pied Piper of Hamelin story. Other novels in this small group include After Hamelin by Bill Richardson and the Keys to the Kingdom series by Garth Nix that starts with Mister Monday (Keys to the Kingdom, Book 1).
When the Pied Piper enchanted the children of Hamelin and led them away, Mari and her brother, Jakob, followed his song. Now they are trapped in a beautiful but cruel world inhabited by a horrid Beast. Finding a way to escape will require some wild magic, in this powerful story of a family torn apart by tragedy, and the magical adventure that heals them.