Changes: Book 3 of The Collegium Chronicles (Valdemar) by Mercedes Lackey
My ALL time favorite fantasy series is the Valdemar series by Mercedes Lackey. I continue to re-read each book and snap up each new offering. Changes is as well written and enjoyable as the 35 or so books before it.
This series could almost be classified as YA even though it deals with adult topics. Sex is inferred never described.
Mercedes Lackey usually writes in 3 book arcs and you can read these books stand-alone but they are so much better if you read them in order.