A Friend in the Dark - An Auden & O'Callaghan Mystery, #1

Rufus O'Callaghan has eked out a living on the streets of New York City by helping the police put away criminals as a confidential informant. But when Rufus shows up for an arranged meeting and finds his handler dead, his already-uncertain life is thrown into a tailspin. Now someone is trying to kill Rufus too, and he's determined to find out why. After leaving the Army under less than desirable circumstances, Sam Auden has drifted from town to town, hitching rides and catching Greyhounds, until he learns that a former Army buddy, now a police detective in New York City, has died by suicide.
Sam knows that's not right, and he immediately sets out to get answers. As Rufus and Sam work together to learn the truth of their friend's death, they find themselves entangled in a web of lies, cover-ups, and accelerating danger. And when they witness a suspect killed in cold blood, they realize they're running out of time.