Across the Great Barrier — Patricia C. Wrede

Eff is an unlucky thirteenth child - her twin brother, Lan, is a powerful seventh son of a seventh son. And yet, Eff is the one who saved the day for the settlements west of the Great Barrier. Her unique ways of doing magic and seeing the world, and her fascination with the magical creatures and land in the Great Plains push Eff to work toward joining an expedition heading west. But things are changing on the frontier.

There are new professors of magic for Eff and Lan to learn to work with. There's tension between William and his father. And there are new threats on the frontier and at home. To help, Eff must travel beyond the Barrier, and come to terms with her magical abilities--and those of her brother, to stop the newest threat encroaching on the settlers.

With wit, magic, and a touch of good pioneer sense, Patricia C. Wrede weaves a fantastic tale of the very wild west.