Animal Instinct by David Rosenfelt

When Corey Douglas hears that Lisa Yates has been murdered, he feels immense regret that he wasn't able to do more for her other than file a report when he suspected she was a victim of domestic violence. Corey was a cop at that time, but now is a private investigator, along with his partners Laurie, Marcus and Simon (his K-9 dog). Corey decides he must find who killed Lisa, and the rest of his team agree. Corey believes that Lisa's boyfriend, Gerald Kline, was responsible, and when Gerald himself turns up dead, Corey is the prime suspect. Corey fights to clear his name and discover both Lisa's and Gerald's killer. Animal Instinct is the second book in the K Team series, an offshoot of Rosenfelt's long-running Andy Carpenter books.