The Disunited States of America by Harry Turtledove is an alternate history tale in which the Constitution of the United States was never written. The resulting fallout is that the "united states" become the "disunited states," with each state going down its own road. Advances in society, technology, etc. all occur at different rates within each state. Some still have slavery. Others have achieved the relative amount of equality we enjoy ourselves. Still others have reversed the white/black dichotomy altogether; blacks are masters over whites. War amongst the states is frequent. California is one of the most advanced and powerful of the states; no one messes with them.
Beckie lives in this alternate world. Justin, a Crosstime Traffic traveler, is from our timeline, but he comes to this variation of the U.S. with his mother on a sort of educational fieldtrip. Justin and Beckie, both teenagers, meet and hit it off. Chaos ensues as they find themselves mixed up in an escalating war between Ohio and Virginia. For Beckie, it's about surviving so she can return to her family in California. For Justin, it's about getting back to his own timeline.
The Disunited States of America flows along well enough, but for all the premise of travel between alternate dimensions, not much is really done with it. Justin arrives, he has some adventures, he leaves. But it's a quick read, which explains why I finished this novel at all (alternate history is not my usual thing) despite there not really being a lot of science fiction meat to chew on. I almost put it down, but at that point I was so close to the end I went ahead and pushed through.
The Disunited States of America is the fourth book in Turtledove's Crosstime Traffic series. If you are looking to start with this series, perhaps you’d be better served by starting with book one and going on from there.