Glasruhen Gate, by Catherine Cooper, is the sequel to Cooper's award winning (and currently free)
The Golden Acorn. It's currently on sale for 99 cents in the Kindle store.
Book Description
When Jack Brenin found out that, beyond doubt, he was 'The One' written about in ancient prophecy, his whole life changed. Now, armed with the golden acorn, his wand and newly acquired skills, Jack must fulflil his promise to ensure that Glasruhen Gate is opened before it's too late. His new friends are depending on him. Jack doesn't want to let anyone down but he knows that he has to be courageous and overcome his doubts if they're to succeed. Jack's extraordinary magical adventure continues.