The Way of Shadows By Brent Weeks

(Night Angel # 1)

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a young boy trains under the city's most legendary and feared assassin, Durzo Blint.

For Durzo Blint, assassination is an art -- and he is the city's most accomplished artist. 

For Azoth, survival is precarious. Something you never take for granted. As a guild rat, he's grown up in the slums, and learned to judge people quickly -- and to take risks. Risks like apprenticing himself to Durzo Blint. 

But to be accepted, Azoth must turn his back on his old life and embrace a new identity and name. As Kylar Stern, he must learn to navigate the assassins' world of dangerous politics and strange magics -- and cultivate a flair for death