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The text on that page roughly translates as:
Law and Official Gazette of the occupied territories of Belgium
laws, regulations and notices
from 1 january to march 31 1917
(from nr. including 295 to 327)
published by the political department at the Governor General in Belgium. To be obtained in Belgium by the print shop of the Act and Official Gazette, Brussel, lowenerstr. 40 in Germany and in neutral countries by the German post offices
This is a journal of the "laws, regulations and notices" passed by the Germans controlling Belgium. If you look at the photo again you'll notice that there are two stamps in the top left corner by the General Staff Library in the War Office on 30 April and 17 October 1917, which means we know this books was printed and published during the war. This begs the question: How did the British manage to get a copy of this while the war was still being fought? Any ideas?