All the King’s Soldiers by John Anthony Miller

A historical action romance

Lisbon,1940:

With Europe in turmoil and France falling, British agent Taylor Hartridge obtains the Nazi invasion plans for Britain. He suspects a double agent in the Lisbon Embassy and insists on taking the plans to London, but is murdered before his departure, the plans stolen.
London analyst Simon Cole arrives in Lisbon, woefully unprepared but willing and determined, pitted against a German spy called the Chameleon, a master of disguise. Racing to recover the plans before the invasion is launched, Cole hunts for Hartridge’s killer while exposing the double agent in Lisbon—even if it’s the woman he loves.