The Thirteenth House is a sequel to Mystic and Rider in Sharon Shinn's The Twelve Houses series. While it can be read on its own, some of the minor characters here - and their powers - are better defined in the earlier book. In this series, the land of Gillengaria is dominated by twelve great houses, some of which may be scheming to take power when the aging king dies. This is the standard fantasy background of a medieval kingdom except for female equality and mystics, who are born with magic powers (sort of a fantasy version of the x-men's mutants). In the first book, a powerful mystic named Senneth (with fire powers) gathered a team of mystics, including Kirra, a shape changer, and king's riders, to gauge the level of support for the king's family.

(Source: Ace Hardcover)