So this was much more interesting than I had initially thought when I started reading it. What at first seems like your standard YA light fantasy about a boy with a magic coin whose wishes come true slowly reveals itself as a light sci-fi story about parallel universes. The stakes were quite life-and-death, with numerous character deaths (view spoiler)[ which, for parallel universe reasons, are both affecting but also non-permanent for our core characters (hide spoiler)]. Also, Nate/Nathan/Nathaniel was fairly odious in most of his analogs, which made me question Ephraim's loyalty to him. 
 But basically, this is a really smart and interesting and fast-paced book with an all-too-rare these days boy protagonist. I think teens would quite enjoy this one.