Everything you need to understand or teach By the Sword by Mercedes Lackey.
The book's major theme is the necessity of honoring one's own calling and talents. This is shown primarily through Kerowyn's life and decisions.
When, at the very end of the story, she is "chosen" for new tasks in the realm of Valdemar, the theme takes on another aspect. Only through her faithfulness to earlier commitments does this chance become possible. While it promises an end to the hardscrabble life of a mercenary and...