Winner Take All, by Barry Eisler

The hero of this novel is an assassin, John Rain.   He was hiding out in Brazil when the CIA get him back to work by offering a hefty fee to kill an Arab gun supplier.   The action moves between Brazil, Hong Kong, Tokyo and Macao.

This is a superficial novel, but a reasonably enjoyable fast read.

Winner Take All (previously published as Rain Storm/Choke Point)