"A Drink Before the War" by Dennis Lehane
First novel that Lehane wrote, and the first book about Patrick Kenzie and Angela Gennaro, who are two young Boston PI's. They are hired by three local politicans to find documents that a cleaning woman is suspected of stealing. They quickly track Jenna Angeline down, and find she has no documents but does have a husband and son who are head of rival gangs. She also has a photo of one of the politican with her husband in a hotel room, with promise of more photos hidden somewhere.
Jenna is gunned down on the street, while delivering the first photo to Patrick, and his picture is on the front page as the hero who shot the killer. A gang war breaks out, as the two detectives try to survive with both gangs after them.
We get a lot of gunfire, but we also get a lot of background on Patrick's abusive father, who was a hero fireman. We also get Gennaro finally deciding she can't take her abusive husband any longer. The crucial finale plays out under an overpass, as the two have to face the husband, and the decision they make will ripple into the future books. There's also a final face off with the politicans that is somewhat satisfying.