The Golden Acorn by Catherine Cooper is currently free in the US Kindle store. (It may in the UK one, too, but the prices are invisible to me there today.) This book is relatively unknown in the US but has garnered 50 reader reviews in the UK store with an average rating of 4.5 out of 5 stars as well as some awards there. It is a children's book with many legends and folklore references, including a main character named Jack. It is also available in paper. I haven't read it myself but I certainly have downloaded it for future reading!
Book description from the publisher:
When Jack Brenin finds a golden acorn lying in the grass, little does he know that it is the beginning of a thrilling and magical adventure. Just an ordinary boy, Jack has been chosen for a hugely important task, and enters a world he believed only existed in legend. Full of twists and turns, talking ravens and mischievous Spriggans, 'The Golden Acorn' is a hugely entertaining and exciting tale from a very talented new author. Your kids will love it, and so will you! This brilliant story deservedly won the Brit Writers' Awards 2010 for unpublished writers and has now been published by Infinite Ideas.
The much awaited sequel 'Glasruhen Gate' was released in February 2011.