A Friend in the Fire - An Auden & O'Callaghan Mystery, #2

After solving the mystery behind the death of his former friend in July, ex-Army Sam Auden has been aimlessly wandering the country. Everything had gone sideways in New York City, so when his phone rings three months later, the caller is the last person Sam expected to be asking for help. Confidential informant Rufus O'Callaghan has been struggling. His NYPD contact was murdered over the summer, and the man Rufus is head over heels for was driven away by his own undiagnosed trauma.
But when he receives an anonymous letter that promises information on his mother, life goes from dark to dangerous in the blink of an eye. Sam and Rufus must dig into Rufus's rough and turbulent past in order to solve a series of contemporary murders connected to his mother. And if the two can't expose who the killer is in time, they will most certainly become his next targets.