Killer With a Key, by Dan Marlowe (epub/Kindle/pdf)
The Kingdom of Killain -- that's the Duarte, a big-city hotel at the crossroads of the world. The grifters, tough lads, girls on the make -- all learn to stay away from Hotel Duarte because Johnny Killain's in charge there. That's his turf -- a flick of his fist makes broken guys and dolls. So Johnny patrolled the dark corridors in peace...until the night he rounded a bend and looked murder square in the eye. The blonde lay on the bed in 609, her face a puffed, blue, strangled horror. Her name was Ellen Killain, and she was Johnny's ex-wife. His still-beloved ex-wife.