Clever Cintha - Robert L. Collins

Though she's the daughter of a blacksmith, Cintha grows up learning to read and think. It's those skills that allow her to trick a wizard into hiring her as a boy servant. It's those skills that help her learn magic.

She also learns that her master is a wicked man. In dealing with him as a young woman she reveals her talents. That brings her to the attention of the King. As she's not been taught under the law, the King tells her she can't be a lawful witch. He does think she can be helpful in other ways.

That other way ends up being an emissary to a neighboring kingdom. Will going to that kingdom allow Cintha to discover her place? It will if she's truly clever.