K-Street by M.A. Lawson (Kay Hamilton #3)

M.A. Lawson is the pen name of Mike Lawson, an established author of the Joe DeMarco series, an excellent series about the unsung aide to the Speaker of the House, who’s always cleaning up the Speaker’s messes … and one of the only protagonists I know who doesn’t carry a gun.
Kay Hamilton is his first female heroine and DOES carry a gun. K-Street is the 3rd book in the series. She’s tough-talking, takes no prisoners and is not easily dissuaded from whatever course she pursues.

“It’s been almost a year since Kay Hamilton was fired from the DEA for going rogue. Since then, she’s been employed by the Callahan Group, a covert intelligence agency based in Washington, D.C. Her job description is as dubious as the people she works for, and the undercover mission that nearly killed her in Viking Bay has Hamilton questioning the legitimacy of her employers.”
“When Hamilton arrives at the Callahan Group’s K Street office to tender her resignation, she unwittingly interrupts a deadly heist during which the robbers have stolen the company safe and left her boss gravely injured. She knows that Thomas Callahan doesn’t keep much cash in the safe—the men must have been after something other than money. But before Callahan slips into a coma, he whispers a name that will lead Kay to an organization even more secretive than the Callahan Group: the NSA.” [Amazon]

As I always suggest, start with the first book in the series, Rosarito Beach.