Gunpowder Empire

Gunpowder Empire is an alternate history novel by Harry Turtledove. It is the first part of the Crosstime Traffic series.
Plot summary
In the novel, Jeremy and Amanda Solter are two teenagers living in the late 21st century. Their parents work for Crosstime Traffic, a trading company using time travel to go back and forth from parallel versions of Earth to trade for resources to help sustain their version of Earth. One summer, the children work with their parents, going to Polisso – in our timeline a village in Romania with ancient ruins nearby, in the alternate timeline a major city of a Roman Empire that never collapsed.
In the intervening centuries, the Romans advanced to the extent of inventing gunpowder – hence the title of the book – putting their armies on about 17th Century level. By 2100, they had not, however, gone through an industrial revolution and much of their social institutions, in particular slavery, remain as much as they...