The Snow Queen by Mercedes Lackey

Aleksia, Queen of the Northern Lights, is mysterious, beautiful and widely known to have a heart of ice. No one would seek her wisdom except as a last resort. But when she's falsely accused of unleashing evil on nearby villages, she realizes there's an impostor out there far more heartless than she could ever be.

And when a young warrior following the Tradition disappears, leaving his sweetheart and mother to fear the worst, Aleksia's powers are needed as never before.

Now, on a journey through a realm of perpetual winter, it will take all her skills, a mother's faith and a little magic to face down an enemy more formidable than any she has ever known..