The Tourist (Milo Weaver, Bk. 1) by Steinhauer, Olen

Milo Weaver used to be a "tourist" for the CIA - an undercover agent with no home, no identity - but he's since retired from the field to become a middle-level manager at the CIA's New York headquarters. He's acquired a wife, a daughter, and a brownstone in Brooklyn, and he's tried to leave his old life of secrets and lies behind. However, when the arrest of a long-sought-after assassin sets off an investigation into one of Milo's oldest colleagues and exposes new layers of intrigue in his old cases, he has no choice but to go back undercover and find out who's holding the strings once and for all.