The Thirteenth Child, but I'm guessing it was the library book sale. This is the first in a trilogy called Frontier Magic by Patricia C. Wrede. I'm pretty sure this falls into the YA category; it was a fast read, and our heroine is a young woman. At least she is by the end of the story. A large portion of this book followed the young Francine--known as Eff--from her early life in Pennsylvania town, to her new how in a frontier town of an alternate 1860s America. An America where magic is commonplace, and in fact most folks need it, or believe they do, in order to deal with the magical wildlife around them.
Eff lives with the stigma of being a thirteenth child, but she is also the twin sister of the seventh son of a seventh son, which sort of makes up for that. But only in some ways. Eff has to find her own way, without the natural benefits her twin brother enjoys. But her bother does love her, and comes to her aid if ever anyone gives her trouble because of her birth order.
By the last third of the book or so, Eff has become a young woman of 18, and has lengthened the hem of her skirts, and begun to put her hair up. I would guess that the remaining books in the trilogy will focus on her at about this age. And I also expect that her romantic impulses, and an emerging magical quality of her own, which both began to blossom by the end of this book, will continue as the main plot points.
Wrede isn't writing the Great American Novel here, so I don't expect all of the characters and sub-plots to be fully developed, but I did feel like some things just weren't important enough to explain, and I was left guessing for myself. I took a look to see if my library has the second and third volumes; they don't. The second volume is available via inter-library loan, but the third volume isn't available in any of the libraries in network. That makes me think that this trilogy isn't all that popular, so I don't think I'm going to bother.