Fleischer, Wolfgang Feldbefestigungen des deutschen Heeres, 1939–1945 (Podzum-Pallas Verlag, Wölersheim-Berstadt, 1998). A reprint of a 1944 German field fortifications manual with additional background and introductory material added. The text is in German.
German Field Works of World War II (Bellona Publications Ltd., Bracknell, Berkshire, 1969). This is a reprint of a 1942 German field fortifications manual. It contains German text with annotations in English.
Kaufmann, J.E. and Kuffmann, H.W. Fortress Third Reich: German Fortifications and Defence Systems in World War II (Da Capo Press, Cambridge, MA, 2003).
War Department (US Army) Handbook on German Military Forces, TM E-30-451, 15 March 1945. Several reprints have been published in past years.
Wray, Timothy A. Standing Fast: German Defensive Doctrine on the Russian Front During World War II: pre-war to March 1943. Research Survey No. 5 (Combat Studies Institute, Ft Leavenworth, KS, 1986).