You can tell this is Turtledove's first book in the series, as he hasn't quite got the formula down yet. Jeremy Solters and his sister are stuck in an alternate history where the Roman Empire never fell when their parents suddenly fall ill and leave them to their own devices. The story is good, but Turtledove is more interested in setting up the world than his characters who are irritating Mary Sues with no flaws and perfect ingenuity. He irons these kinks out in the next few. Gunpowder Empire introduces the Crosstime concept but it's hardly a requirement. I'd skip it altogether.