Across the Great Barrier. Like Summers at Castle Auburn, they're ones that I keep thinking about fondly, and seriously consider re-reading Right Away rather than Eventually. (I've nibbled at Summers, and did re-read Thirteenth Child pretty soon after.)
Thirteenth Child is set in 1800s America, where the frontier is filled with magical monsters instead of Indians*. It's an alternate universe, but not the sort where one thing changes and everything else diverges from that; it's always been different, and always been similar. Washington and Jefferson were still presidents; Jefferson and Franklin were wizards; the Civil War still happened, but thirty years sooner; slavery still existed, but apparently Africa was by and large still powerful enough that they were the ones to colonize South America.
The main character is Eff; she's the thirteenth child in a family that was trying for a seventh son, because the father is himself a seventh son. The seventh son of a seventh son is a powerful magician. Unfortunately, the common wisdom is that the thirteenth child is, at the least, bad luck; some people (like Eff's uncle) even believe them malicious. It doesn't make for a happy childhood for Eff, until her family (minus the oldest, grown children) moves to the frontier, where nobody knows how many kids there were or what order they were born in.
But Eff still knows. She's constantly questioning her own motivations: is this the sort of thing I would do if I were starting to go bad? It's really interesting to watch Eff and her twin, Lan, grow up under the influence of What Everybody Knows about thirteenth children and double-seventh sons.
And I liked the American setting. It really felt like an American fantasy rather than a European/medievalish one, and that's a nice change.
* This is the only thing that disturbs me about these books: no Indians. Their massacre is one of the great American sins, and it bugs me that it's eliminated in this alternate history not because the colonists chose a better path, but because the victims don't exist. Did I miss them? Do they exist in the world, and merely weren't involved with these particular stories? Are they supposed to not be around because the monsteredness of middle America held them West? Whatever the reason, their absence bothers me.