A Warp in Time (Horizon, Book 3)
There are other kids, too...
The seven-book mega-series begun by Scott Westerfeld (Uglies) reaches a pivotal turning point as only New York Times bestseller and National Book Award winner Jude Watson could deliver!
There is an online gaming component to these books, so many of the children's adventures have a distinct video game feel-- they travel around, have to fight monsters, and find gadgets that help them with what they are attempting to do. The first book in the series is by Scott Westerfeld, and the second by Jennifer Nielsen, but Jude Watson is able to maintain the same feeling and writing style that made these books quick reads.
Children who love science fiction series like Voyagers! Or Todd Strickland Mars: Year One will find plenty of action and adventure in the Horizon series, and the mystery will appeal to those who have finished The Thirty-Nine Clues or Infinity Ring series.