Nightrunner (Vol. 3): Traitor's Moon
Skala is at war and, without the help of its mysterious neighbors, the Aurenfaie, it will surely lose, so the dying queen sends her youngest daughter, Princess Klia, away with an exiled 'faie named Seregil to plead their case. The crown princess, Phoria, is not pleased that her younger sister was chosen for such an important task. Klia herself, though eager to aid her mother, fears for Skala.
Unlike the first two books in this series (and, indeed, like books in most fantasy series) Traitor's Moon stands alone. Characters are clearly introduced. This is probably its greatest strength as a book, though it has many to choose from.
Traitor's Moon is nowhere near as light as the other books in the series, but the characters remain the same even though the situations change.