The High Priest and the Idol by Jane Fletcher

Jemeryl and Tevi's relationship is put to the test when the Guardian sends Jemeryl on a mission that lands her not only in harm's way, but also back into the sites of a previous lover. The Protectorate of Lyremouth promises liberty for all its citizens, but this does not mean that everyone is equal. When the Jemeryl is summoned alone to Lyremouth, she suspects a ploy to separate her from her lover, Tevi. After all, many disapprove of their relationship--not because they are both woman, but because Jemeryl is a sorcerer and Tevi is not. The objections reflect the distrust, disdain, and outright hostility across the social divide, between those who can work magic and the ungifted majority. The task Jemeryl is given, to track down an ex-lover who has turned renegade on the Coven, does nothing to assuage her doubts. However, old bonds of affection are enough to make her accept the assignment, even though she is sure that she has been told only half the true story. But Jemeryl is wrong--she has not been told even a tenth of what is behind this mission. When Jemeryl does not return, nobody can stop Tevi going after her. Book four in the romantic fantasy series--Lyremouth Chronicles.