A third book comes to mind; but finding relevant data, would require a visit to the library, and the item chancing to be present there on the shelf. Will try to make that errand when circumstances permit.
Book concerned, has been found again. It's "The Secrets of Rue St. Roch" by Janet Morgan, publ. 2004. Its interest re subject of this thread, is that it concerns a World War I intelligence-gathering operation centred on an Allied agent who was a citizen and resident of Luxembourg -- a country (small, and 100 years ago rather obscure) which I perceive as rarely heard-of in a WWI context.
Book has some content of interest re Luxembourg's WWI situation and experiences. The then-reigning Grand Duchess being pro-German, she was in August 1914 a "compliant victim" as regards invasion and occupation by Germany. In consequence, Luxembourg received (very relatively) gentler treatment at German hands, than did other occupied territories. However, book indicates that notwithstanding, things there were fairly grim for the populace. (And, it is recounted, some 10% of the male population of military age, sneaked off to enlist in the Allied forces -- any who fought for Germany, are not mentioned.)
Despite the above-mentioned, I don't feel able to recommend this tome very warmly. The large majority of its 350 pages is taken up by the narration in minute detail, of the lengthy genesis and setting-up of the operation in question (which did not actually get into action until early 1918), and the convoluted and sometimes unharmonious interaction between the many different Intelligence folk involved. Material telling of how things were in occupied Luxembourg (and, a little, in wartime Germany itself) represented, for me, small nuggets of interest among a great deal of not-much-interest -- and IMO the author writes in a dull, plodding style. The book would possibly be fascinating to a "spooklore" devotee -- which I am not, and in the main I found reading it, something of an ordeal.
