The Left Behinds: The iPhone that Saved George Washington, by David Potter (Crown Books for Young Readers, Jan. 2015).
For the Left Behind kids, stuck spending Christmas at their boarding school, going to see the re-enactment of Washington Crossing the Delaware was mostly something to be endured. (Except for Mel, the history geek of the three of them, who was at least a little interested),
But then Mel find General Washington dead (really truly dead) in a stable. And he realizes (in bits, as is only to be expected) that he's actually travelled back in time. Bev and Brandon come back in time too, and Mel convinces them that they have to come to the rescue of the colonies, or the United States will never exist.
Thanks to texts from his history teacher back in the present, Mel realizes that his iPhone was the time travel catalyst. But unless he can charge it, the kids won't get home again.
So there in the middle of colonial winter (with Washington dead in barn) the three set out, braving hostile Hessians and suspicious colonial kids to find Ben Franklin, the only iPhone charging hope going. But once that's done, there's the matter of George Washington...so Mel hops back in time again, to foil the killers...
And it's fine history adventure fun. I'm a smidge doubtful about the appeal of the American Revolution to modern kids (mine are tired of it), and I think the blurb at Amazon goes over board--"Percy Jackson fans will embrace this humorous time travel adventure..." (what? P.J. is totally utterly different, except that he, like Mel, is a boy thrown into adventure, but of a totally different sort). This is more along the lines of Jack and Annie from the Magic Tree House books for older readers, and it's done well, with bits of humor, and vivid descriptions. But it's not Percy Jackson.
Time travel via iPhone app is certainly an interesting premise, and there's a not uninteresting story about why it happened to the kids (there's a villain who wants to unleash temporal chaos on the world). There's certainly lots of room for more stories. Despite the premise, there wasn't much overt concern for the paradox that beset time travelers, which, since the characters were busy staying alive and keeping Washington alive, is understandable, but which may disappoint time travel fans. I'd classify this one as "time travel as a mechanism that allows modern kids to have adventures with historical figures" story, as opposed to "time travel as an opportunity to reflect on a different culture/time and see how it changes the protagonists." And I'm really more a fan of the later.
Something I found interesting was that although the three kids weren't friends at the beginning of the book, they did Not grow to appreciate each other much, and did not become inseparable comrades by the end of it. Points for realism! But on the other hand, because there wasn't much change or growth to the characters, even though we learn more about their lives, there wasn't much opportunity to care that much about them.
In short there wasn't anything I actively didn't like about the book, but things just didn't feel particularly magical to me, and so it was a book that I read perfectly happily, but didn't love.