Abacus

Abacus is a middle-grade novel that whisks readers along with AP, the 12-year-old protagonist and science geek, who travels through time with Kate, his oh-so-bored 15-year-old sister. Their journeys to Arthurian England, the Wild West, and Ancient Egypt, get them into some very tight spots, but AP uses his broad knowledge of science to help them escape. And stalking them on every trip is a hooded stranger whose attacks show he'll do anything to stop them.