Turtledove is another author like Heinlein; the rule is that if a book is less than an inch thick, I'll like it.
These two are the latest installments of the "Crosstime Traffic" series, following Gunpowder Empire and Curious Notions. All the books entirely stand alone.
They're essentially YA, though not published that way. They all have (different) young adult protagonists, who go to (different) alternate timelines that Crosstime Traffic is exploiting, and run into (different) problems. These two also have protagonists within the world visited, which I don't remember from the earlier two, but it's been a while and they're the kind of thing I read like candy and don't retain well.
In High Places concerns Annette, aka Khadija, a girl a few months away from going to Ohio State, who is meanwhile working with her parents in the Kingdom of Versailles, in an alternate world where the Black Death killed two-thirds of Europe and everything is still dirty and medieval. She meets Jacques, a young man of the period, and they are captured by bandits and enslaved. Things then get really interesting, though I wasn't entirely convinced that the cross-time slavers motivation was sufficient and made sense. Still, it did provide an and interesting extra dimension. It's a fast-paced read, and a lot of fun.
The Disunited States of America concerns Justin, again a boy just ready for college in the original reality, and Beckie Royer, a girl from California in the alternate. This US hasn't stayed together because of some problem ratifying the constitution, and Ohio is at war with Virginia, and has been arming the, well, my goodness, with weird coyness Justin thinks of them as African-Americans and the people he talks to about them call them a word he can't bring himself to say, but which is mentioned without being mentioned every time. This book does not contain "the n word" but it sure makes you think about it a lot. I suppose this was a good choice, for a YA audience, but I found it distracting and bizarre. This is also a fast-paced read, but I found it less fun, without having any more depth. Or maybe I just read them too close together.
I'd recommend these to people who really like alternate histories, to young adults, and to people convalescing from the flu. I'll buy more to keep around for this purpose.