The title of this book should be “Tolkien: A Biography Through World War I”, because it’s not just about his wartime experiences, but about his life up till about age 26. It focuses on his closest boyhood friends and their fates in the war.
Garth He goes into detail about the development of Tolkien’s languages and the associated world building. He traces elements of Tolkiens legendarium to his experiences in the trenches, but it’s not a simplistic one-to-one mapping.
This is a great book that should be read by anyone interested in Tolkien. My only complaint is that it stops in 1918.