Time Trapped — Richard Ungar

Caleb thought he'd escaped Uncle's clutches and could have a normal life in 1968, but no such luck. After being forcibly returned to Timeless Treasures and his old job of stealing valuable objects from the past, he learns that things have gotten even more sinister. Training the new kidnapped recruits doesn't seem very important to Frank, Uncle's evil lackey, even though a few of these kids have amazing theiving skills and genius for new technology. But then Caleb figures out it's because Frank doesn't plan on keeping them around very long - or keeping them alive.

Stakes are high for all of the time snatchers. If only Caleb can convince the new ones to stop having fun with the technology and use it to save their own lives.