The Defector by Daniel Silva

Gabriel Allon is pulled out of semi-retirement when Russian defector Gregori Bulgavov disappears from his comfortable life in London. The British are willing to believe Bulganov re-defected, but Allon knows otherwise. Once again, Allon assembles a talented team to free a friend from a seemingly impossible situation. This book is a sequel to Moscow Rules, but can stand alone. I'd recommend reading Moscow Rules first, though. Daniel Silva is the master of the spy thriller, and this one is no exception. I like the earlier Gabriel Allon books best, but as long as Silva keeps writing them, I'll keep reading them.