Harry's War by Harry Drinkwater. It is about WW1 in an Infantry regiment with a surprising amount of aviation mentioned, being observed by captive balloon by day and frequently bombed by night. Almost certainly the best book about the Great War that I have ever read. The author kept a secret diary that he was able to expand to write this book in the 1920s. It was stored in the loft of his house and was not discovered until a few years ago, some 50 years after his death.