I happen to collect military memoirs, particularly from the First World War. That was a big, wide war, and the American imagination of it is limited mostly to France, 1918. Most of the War was not like that, and first-hand stories of German U-boats and commerce raiders, Lawrence's Bedouin revolt, Austro/Russian cavalry battles, Japanese island-hopping in German Micronesia, Zeppelin raids over London, civil war in Russia, German invasion of Ukraine and the Baltics, the Alpini on the Isonzo front and all the rest give an idea of just how revolutionary and huge that conflict really was, and how it was full of heroics and achievement, contrary to the pacifist literature of the 1930s which imagined the entire War as a four year sentence to a lethal mud hole in France and Flanders.
For more modern stories, Robin Moore's "The Green Berets" and Warner Smith's "Covert Warrier" were good recent reads.
I'm also expanding my reading into the post WWII career and certain operations of Reinhard Gehlen, who briefly describes in his memoirs ("The Service") his Organization's support of partisans in Soviet Russia and Eastern Europe, including by trapped and stay-behind German units, well into the 1950s. I'm preparing a research paper on this for publication in a minor journal whose name you probably wouldn't recognize.