The Last Run - Greg Rucka

In the Special Operations Section of the British Secret Intelligence Agency, there are only three ways you leave the job: You're promoted out, you quit, or you die. Tara Chace has been waiting for the first while avoiding the third. After ten years, though, she's getting old, and she's getting slow, and the time has come for her to step aside.

But before she can leave the Section to younger hands, a mysterious message out of the past has Chace embarking on one final run, into the most dangerous field of operations in the world: Iran. Communication from a decades-silent agent brings with it the potential for a devastating blow against a repressive regime. Chace's instructions are clear: Bring the agent out alive. But nothing in service of Queen and country is ever that simple. With allies and enemies alike all serving their own agendas, Chace finds herself alone, hunted-and racing the clock to complete what is destined to be her last run.