Flesh and Fire — Laura Anne Gilman

Once, all power in the Vin Lands was held by the prince-mages, who alone could craft spellwines, and woo selfishly used them to their own gain. Now, fourteen centuries after a demigod shattered the Vine, it is the humble Vinearts who know the secret of crafting spells from wines, the source of magic, and they are prohibited from holding power.

But a new darkness is rising in the vineyards, and only one Vineart, Master Malech, senses the coming danger, He has but one weapon to use against it: a young slave named Jerzy, whose origins are unknown, even to him. Yet his uncanny sense of the Vinearts' craft offers a hint of greater magics within -- magics that Malech must cultivate in his new apprentice before time runs out. For if Jerzy cannot unlock the secrets of the spellwines, the Vin Lands will sure be destroyed.