This time, I have to go with nothing. I've never read a book taking place during the great war. In fact, I generally avoid novels that take place during real wars. I even avoid movies / TV shows / etc. Why? For the most part, I know who wins. Second, these books are always sad and depressing. I have my life for the sad and depressing thing to be in my life, so when I read a book I want something a bit more cheerful. I don't mind fictional wars so much. On the one hand, there the war is actually exciting, since even when the obvious party wins, there's always the how. It also has the comfort of none of that having actually happened, so I don't have to think of that having been an actual person's life. If I want to think about the awfulness of an actual person's life, I have my grandparents for that, who actually lived through both world wars right in the middle of it. Or just look at my country's map, and I'll see the awfulness of it all, since we were on the losing side and lost 2/3 of our territory in WWI.
The only thing that could come close to me reading something that takes place during WWI was this series, which has several seasons during it:
They were sad and depressing episodes more often than not, but by then I was invested in the characters, and couldn't stop watching. However, I did go more slowly than the previous seasons, and took frequent breaks. Especially when I was crying too much.