Requiem for an Assassin by Barry Eisler

Weakest book in the series. Our hero Rain is angry and not so likable. PW: If you had to kill three people to save your best friend's life, would you do it? When John Rain decides to get out of the business, his hand is forced by rogue CIA operative Jim Hilger. Hilger kidnaps Dox, Rain's trusted partner and closest friend, and offers Rain a choice: carry out a final assignment, or bear the responsibility for Dox's murder.