Hard Rain by Barry Eisler

Hard Rain is the second book in the John Rain, hitman, mystery series. In this outing, Rain has gone underground, but he is found by one of his old employers, who wants him to take on a Yakuza killer. Soon, Rain is involved in a chase that brings him into contact with an old love, a new love, and requires all of his skills to survive--and take a little revenge.

John Rain is a hybrid Japanese/American, living in Japan. Rain's descriptions of his hangouts in Tokyo and the neighborhoods he trawls are a big part of the fun in this series.