The books of the series "A photographic Journey of Uniforms and Equipment through the 20th Century" are still available, and more volumes were added in the series (not WW1, but GDR related). A overview of what is released so far. (more in preparation)
Belgium
Volume 1: Belgium 1914-1940 & Congo
Imperial Germany
Volume 2 : Imperial German/KuK steel helmets and their post-war use
Volume 3 : Imperial Germany 1914-1918 : Equipment of the Foot Soldier
Volume 4 : Imperial Germany 1914-1918 : Weapons and Machine Gun
Volume 5 : Imperial Germany 1914-1918 : Technical Equipment : Medical, Artillery, Pioneer, Gas
Volume 6 : Imperial Germany 1914-1918 : Communication Equipment : Radio, Telephone, Signalling
Volume 7 : Imperial Germany 1914-1918 : Enlisted Men's Spiked helmets
Volume 8 : Imperial Germany 1914-1918 : Enlisted Men's Cavalry helmets, Tschako's, Colonial, and Cloth Headgear
Volume 9 : Imperial Germany 1914-1918 : Enlisted Men’s Uniforms
East Germany (SBZ/DDR)
Volume 10 / DDR-1 : SBZ (1945-49), Police (1949-54), Border Guards (1946-61), KVP (1952-56)
Volume 11 / DDR-2 : Nationale Volksarmee (1956-1990), Part 1
Volume 12 / DDR-3 : Nationale Volksarmee (1956-1990), Part 2 – Civil Defense (MdI)
Volume 13 / DDR-4 : Nationale Volksarmee (1956-1990), Part 3 - Border Guards (1957-1990)
Volume 14 / DDR-5 : Police Formations/MdI (1954-1990), Fire Brigades
Volume 15 / DDR-6 : Field Combat Clothing (1950-1990), Drillich, Fallschirmjäger/Aufklärer & Special Forces
Volume 16 / DDR-7 : Civilian Organizations, Part 1
Volume 17 / DDR-8 : Civilian Organizations, Part 2
Volume 18 / DDR-9 : DDR Flags: Civilian, Legal, State, Political and other organizations in the DDR seen through their flags, documents and artifacts.
Volume 19 / DDR-10 : Equipment, gear, weapons
Volume 20 / DDR-11 : Socialist Wall Carpets
Volume 21 / DDR-12 : Socialist Busts, Street Signs and Building Plates