At the City's Edge by Marcus Sakey

Jason Palmer is an Iraqi war veteran who is at loose ends in his hometown of Chicago after being dishonorably discharged. When his brother, Michael, is murdered, Jason is left as the guardian of Michael's son, Billy, and has his eye on vengeance. Michael was a community activist and was possibly working with the police about gang activity in his neighborhood. Jason joins forces with cop Elena Cruz who has something of her own to prove after her affair with a superior has gotten her an awful assignment and lots of ribbing. Was Michael killed by a gang or someone with power in the city? Jason and Elena put their lives on the line to find the truth and closure.