The Traitor's Gold - David Leadbeater

At the root of all riches lies evil. Ex-MI5 operative Joe Mason is on a mission to rescue a friend who has been kidnapped. Meanwhile, ancient coins - remnants of a Chinese casino buried for six hundred years - are starting to surface in Japan's clandestine underworld.
Mason knows the coins and kidnapping are linked. Someone has finally unearthed the long-buried ancient casino, and its riches - both financial and archaeological - are now up for grabs. But hidden forces are desperate to conceal the discovery and take it for themselves, kidnapping and killing to silence anyone in their path.
In a high-stakes battle to reveal the casino to the world, Joe and his team race across the region, from high-rise Hong Kong to Tokyo's bustling markets to the covert gambling rings of Osaka. But they are being hunted by elite forces - and it's only a matter of time before the target on their back catches up with them.
In this perilous quest for the world's oldest casino, Mason must find the site before his enemies find him. But can he unearth the secrets of the gold in time?