Fans of Margaret Peterson Haddix's Shadow Children series will LOVE her new series, Missing, which begins with Found.
When thirteen-year-olds Jonah and Chip, who are both adopted, learn they were discovered on a plane that appeared out of nowhere, full of babies with no adults on board, they realize that they have uncovered a mystery involving time travel and two opposing forces, each trying to repair the fabric of time. The cliff-hanger ending promises that the next book in the series will pick up with the same action and adventure that permeates this book.
The many fans of Margaret Peterson Haddix will love other books by her, including the previously mentioned Shadow Children series (starting with Among the Hidden), Double Identity, Running Out of Time, and my personal favorite Takeoffs and Landings. Another author you may enjoy is Eoin Colfer. And don't forget the ultimate classic of time travel, The Time Machine by H.G. Wells.