Parker, T. Jefferson. Charlie Hood novel.

This time around, Hood is running the California-Mexico border with the ATF, searching for the iron river, the massive and illegal flow of handguns and automatic weapons that fuels the bloody cartel wars south of the border. Gunrunners by nature aren't exactly ethical, but the lengths they'll go to, and the innocent lives they'll risk, are shocking even to Hood. Most shocking of all is the close personal connection Hood finds wrapped up in events south of the border, a connection that shakes him to his core