These are the books by Kate Thompson that are reviewed on this site:
Rilka sets out on a quest to catch a beguiler and to understand its nature. And in doing so she learns a lot about her own nature too.
One problem. Two people trying to solve it. Faery and Ploddy. Course, you'll have to believe in faeries to get on with this one.
A tale of highway robbery, but not necessarily the obvious kind.