Gang warfare gets gruesome in the eighth book of the CHERUB series, which Rick Riordan says has plenty of action.
CHERUB agents are highly trained, extremely talented and all under the age of seventeen. For official purposes, these agents do not exist. They are sent out on missions to spy on terrorists, hack into crucial documents, and gather intel on global threats all without gadgets or weapons. It is an extremely dangerous job, but these agents have one crucial advantage: Adults never suspect that teens are spying on them.
In "Mad Dogs," the British underworld is controlled by gangs. When two of them start a turf war, violence explodes onto the streets. The police need information fast, and James has the contacts to infiltrate the most dangerous gang of all "