The Thirteenth Child by Patricia Wrede

The Thirteenth Child is a fantasy book that takes place in an alternate version of frontier America. This is a magical world containing mammoths, steam dragons, and many unknown and dangerous creatures. In this version of history, Lewis and Clark never returned from their westward explorations, and the only thing that holds back the dangers that lurk on the frontier is the Great Barrier, a magical boundary over which most creatures cannot cross.

This is also the story of a large family. Eff is the thirteenth child, and her twin brother, Lan, is the seventh son of a seventh son. According to European magical traditions, this birth order determines their destinies - Eff as a bringer of bad luck and Lan as a powerful magician. The treatment that Eff receives from her uncles, aunts, and cousins contributes to her family's decision to leave their home for a western frontier community where her father will teach magic at a small university.

The story explores Eff's and Lan's emerging talents and the problems that each of them has as they develop as young magicians in this frontier land. As their education continues, we learn about different magical traditions from other continents and how these traditions may mean that their destinies are not what they originally thought. There are also settlements beyond the Great Barrier, which the twins get to explore when they visit their sister who has joined a settlement of non-magic users trying to cope with the dangers of frontier life in more conventional ways.

This is the first book in the Frontier Magic series from Patricia Wrede. I'm eagerly awaiting the next book!