Gone to darkness : by Barbara Nickless.
(Book on CD)

Iraqi war vet and former railway cop Sydney Parnell is now the youngest homicide detective in Denver's Major Crimes Unit. In the past, gut instinct has served her and her K9 partner well. But it's not a trait Len Bandoni, her old nemesis turned reluctant mentor, admires. Not until Sydney's instincts lead to their first case: a man tortured and beaten to death, then left in a refrigerated train car with cryptic messages carved into his body. The victim is a well-liked member of an elite club called the Superior Gentlemen. At first glance, the club appears harmless. But beneath its refined surface swim darker currents. As Sydney; her K9 partner, Clyde; and Bandoni investigate the grisly murder, the three develop a bond that carries them through a shocking series of crimes and a horrifying conspiracy that threatens the detectives' lives and promises to bring their beloved city to its knees.