James, Peter, 1948- Roy Grace series.

"New York, 1922. Five-year-old Gavin Daly and his seven-year-old sister, Aileen, are boarding the SS Mauretania to Dublin--and safety. Their mother has been shot and their Irish mobster father abducted. Suddenly, a messenger hands Gavin a piece of paper on which are written four names and eleven numbers, a cryptic message that will haunt him all his life, and his father's pocket watch. As the ship sails, Gavin watches Manhattan fade into the dusk and makes a promise, that one day he will return and find his father"--Dust jacket flap