I am sure a great many people will agree with me when I say 'The First Casualty' by Ben Elton. Why? I'd read the whole book without being able to deduce whether it was intentionally comedy or a serious piece of Literature. Anyway, I'm sure it's been derided enough times already on this Forum, so I'll say no more.
Now, if we could just deviate from the original topic title ever so slightly and alter 'book' to 'poem'...
It's not specifically a First World War poem, I don't think - more a general 'war poem'. It's written by a woman named Gillian Clarke, who would, I suppose, be termed a 'modern poet.' I've been to hear her read aloud from her own collections live, and didn't find her at all engaging, to start with. Yet in spite of myself, due to my interests, when she told us she had been asked to write something on the Vietnam War, (at least, I think it was the Vietnam War - I might be wrong) my ears pricked up. The first stanza described how she was out in the countryside, watching a combine harvester at work, when she came upon some dead field mice. The remainder of the poem proceeded to liken the fallen soldiers to the mice, because they had been frolicking in a field, and could not have been more unaware of the danger about to befall them as they were caught up in the blades. Apparently, these mice also looked so 'innocent', and at one with nature, as it were. Anyway, I couldn't help feeling a pang on the soldiers' behalf, and on behalf of all soldiers really (not, of course that I have a right to feel that way, having never served personally) but, for starters, Gillian Clarke had no specific interest in her subject matter, beyond that she had been 'asked' to write because some friend of hers deemed it appropriate. To say the least, I thought the poem rather pathetic, lacking in substance and unworthy of it's subject.
As well, I have to admit being not particularly fond of Vera Brittain's 'Testement of Youth.' She does come over as being too ambitious, quite pretentious, and always, ALWAYS this need to be unconventional! (That said, the part where she describes sitting in her fiancee Roland's living room with his parents, and his clothes are sent back to her, and she can smell him on them, and she can also smell this other, unidentified, strange smell... the smell of the war... and it's as though the war is an acutal physical character... is harrowing). I also thought it read more like a philosophy textbook than a forthright autobiography.
Anyway, I have said rather more than I intended there