This site contains some v. useful reference books on various Great War topics. Available in English and German. A bit of clicking around reveals some splendid stuff.
As is customary with publications from this establishment, they are, unfortunately, eye-wateringly expensive.
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"Sometimes things that are not true are included in Wikipedia. While at first glance that may appear like a very great problem for Wikipedia, in reality is it not. In fact, it's a good thing." - Wikipedia.