I would like to nominate Goodbye To All That by Robert Graves. I think it would be an interesting companion piece to Sassoon's war poems. From Kobo:
In 1929 Robert Graves went to live abroad permanently, vowing 'never to make England my home again'. This is his superb account of his life up until that 'bitter leave-taking': from his childhood and desperately unhappy school days at Charterhouse, to his time serving as a young officer in the First World War that was to haunt him throughout his life.
It also contains memorable encounters with fellow writers and poets, including Siegfried Sassoon and Thomas Hardy, and covers his increasingly unhappy marriage to Nancy Nicholson. Goodbye to All That, with its vivid, harrowing descriptions of the Western Front, is a classic war document, and also has immense value as one of the most candid self-portraits of an artist ever written.