Death in the Kingdom by Andrew Grant

British agent Daniel Swann fled Thailand after murdering the son of the Kingdom's top underworld boss. Now he is back, ordered by his government to recover a small black box from the bottom of the Andaman Sea. Business as usual, he doesn't ask questions. But as his friends are beheaded one by one and he is pursued by CIA agents, Swann realises his mission has become personal. Someone wants him dead. Trying to stay alive on the streets of Bangkok, Swann engages in a deadly game of cat and mouse. When he finally discovers the contents of the black box, he is caught up in a government-level conspiracy. But with his enemy always one step ahead, Swann knows he is being betrayed. So he turns to the only people he can trust—the underworld. This is the first book in the Daniel Swann series.