Night of the Fox by Jack Higgins

On a secret sea manoeuvre just before D-Day, American Colonel Hugh Kelso goes down in the English Channel. Wounded and adrift for days, he washes ashore on the German-occupied island of Jersey. The news spreads panic through the Allied high command: Kelso knows the time and place of the invasion. He must be rescued — or silenced. A British professor turned Nazi impersonator and a young Jersey girl posing as his mistress set off to find Kelso in the fiercely guarded island fortress. The pair join a deadly game of wits that they must win….or perish in the darkness of the “Night of the Fox”