Russ Schneider.

Russ Schneider was an extraordinary writer and his first novel, Siege. A Novel of the Eastern Front, 1942, is unlike any other war book. He taught himself German, the better to research German materials, and produced one of the masterpieces of war literature.

The link will tell you where you can buy this book about the German defense of Cholm and Velikiye Luki, both besieged by the Red Army.