I read this book not long ago; I know it's a "children's/YA" book, but I really enjoyed it anyway. I think it's a nice mix of real-life family problems among siblings and a very magical world that one of the siblings enters. I read this right before going to Wales, where the book takes place, and it made my trip that much more "magical."
Relates what happens to three American children, unwillingly transplanted to Wales for one year, when one of them finds an ancient harp-tuning key that takes him back to the time of the great sixth-century bard Taliesin.