Unmage

Timo Valendi's place in Mage Guild-the most powerful of all of Tregella's guilds-seems secure. His mother is a magically formidable and politically important Mage and he's apprenticed to the Mage Guild Primus, the head of the guild and the most influential man in the country.
But Timo has a secret, one neither his mentor nor his mother want to know. Like Kara Fonti, the half-sister he is forbidden to see, Timo has unmagic-a rare ability that allows him to see and manipulate the spells others have created. But unlike his sister, Timo is also a powerful mage.

When he no longer can count on the protection of his mother and the Mage Guild Primus, Timo is forced to contend with the corruption that lies at the heart of Mage Guild. Even help from his sister Kara and her unguilded allies can't guarantee his safety. Timo must use all his skills and abilities to save himself-and those he cares for-from Mage Guild.