Imager : the first book of the imager portfolio / L.E. Modesitt, Jr.

Enhanced descriptions from Syndetics:

I mager is the beginning of a whole new fantasy in a whole new magical world from the bestselling creator of Recluce. Although Rhennthyl is the son of a leading wool merchant in L'Excelsis, the capital of Solidar, the most powerful nation on Terahnar, he has spent years becoming a journeyman artist and is skilled and diligent enough to be considered for the status of master artisan--in another two years. Then, in a single moment, his entire life is transformed when his master patron is killed in a flash fire, and Rhenn discovers he is an imager--one of the few in the entire world of Terahnar who can visualize things and make them real.

He must leave his family and join the Collegium of Imagisle.  Imagers live separately from the rest of society because of their abilities (they can do accidental magic even while asleep), and because they are both feared and vulnerable. In this new life, Rhenn discovers that all too many of the "truths" he knew were nothing of the sort. Every day brings a new threat to his life.  He makes a powerful enemy while righting a wrong, and begins to learn to do magic in secret. Imager is the innovative and enchanting opening of an involving new fantasy story.