A Dance of Cloaks (Shadowdance Book 1) by David Dalglish (Second-Hand)

The Underworld rules the city of Veldaren. Thieves, smugglers, assassins . . . they fear only one man. Thren Felhorn is the greatest assassin of his time. All the thieves' guilds of the city are under his unflinching control. If he has his way, death will soon spill out from the shadows and into the streets. Aaron is Thren's son, trained to be heir to his father's criminal empire. He's cold, ruthless - everything an assassin should be. But when Aaron risks his life to protect a priest's daughter from his own guild, he glimpses a world beyond poison, daggers and the iron rule of his father. Assassin or protector; every choice has its consequences. Fantasy author David Dalglish spins a tale of retribution and darkness, and an underworld reaching for ultimate power.