Patricia C Wrede
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 she wants that kind of responsibility. Everyone keeps waiting for her to do something amazing--or to fail in a spectacular way. Worse, her twin brother, Lan, a powerful double seventh son, is jealous of all the attention she's been getting.
Even as Eff protests that she's just an ordinary girl, she's asked to travel past the Barrier Spell with one of the new professors at her father's school. The land west of the Barrier is full of dangers, both magical and wild. Eff will need to use all her strength--magical and otherwise--to come safely back home.
-description taken from goodreads