John Harris 
Fox from His Lair [EPUB ebook] 

A brilliant German agent lies in wait for the Allied invasion of Nazi-occupied France. While the Allies prepare a vast armed camp, no one is aware of the enemy within, and when a sudden deadly E-boat attacks, the Fox strikes, stealing secret invasion plans in the ensuing panic. What follows is a deadly pursuit as the Fox tries to get the plans to Germany in time, hotly pursued by two officers with orders to stop him at all costs.