There are 2 books in the Frontier Magic series. The newest book is Frontier Magic which came out in 2011.

1
Thirteenth Child

Book cover of Thirteenth Child
Eff was born a thirteenth child. Her twin brother, Lan, is the seventh son of a seventh son. This means he's supposed to possess amazing talent - and she's supposed to bring only bad things to her family and her town. Undeterred, her family moves to the frontier, where her…
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2
Frontier Magic

Book cover of Frontier Magic: #2 Across the Great Barrier
From New York Times #1 bestselling author Patricia C. Wrede, the second in the series of magic on the western frontier.

Eff is riding west, away from the safety of the frontier city she's always known.... Eff could be a powerful magician if she wanted to. Except she's not sure…