The Hawk and His Boy by Christopher Bunn

One night in the city of Hearne, a young thief named Jute is instructed to break into a wizard's house and steal an old wooden box. It sounds like a straightforward job. Climb down the chimney, creep through the house, find the thing and get out fast. Unbeknownst to the boy, however, the box contains the knife that killed the Wind. Overcome with curiosity, Jute opens the box and sets off a chain of events that soon has him on the run from the wizard, his old masters in the Thieves Guild, and their client, who happens to be the Lord of Darkness himself. On his odyssey of escape, Jute is aided by an unlikely assortment of friends, including a guilt-ridden assassin, a reluctant wizard, and a hawk who just might be able to teach him how to fly. But the Darkness will do anything to find Jute, even if it means plunging the whole land into war.

The Hawk And His Boy is the first book of The Tormay Trilogy. The trilogy continues with The Shadow At The Gate, and concludes with The Wicked Day.