This book carries a recommendation by A.N. Wilson to the effect that it's "the best book about Tolkien that has yet been written". While I don't think it is actually better than Tom Shippey's work, it is none the less a very good book, moving well beyond the cliches of equating the Dead Marshes to the Somme. It basically concentrates on the story of the friendship between Tolkien and three of his schoolmates, G.B. Smith, Rob Gilson and Christopher Wiseman, who together formed an intimate group called the TCBS. It could have been the story of any group of naive and idealistic young men, pledged to change the world and to renew a sense of old values through their works of literature, except of course that one of them actually did.
Garth saves his analysis of the effect of the war per se on Tolkien's writing for an afterword, and concentrates for most of the book on the narrative of what actually happened to the four friends. This is very effective. The actual events of the Somme are dealt with surprisingly quickly, but Garth manages to balance a detailed account of where Smith, Gilson and Tolkien were (Wiseman was in the Navy) with a sense of the overall perspective of the agonising shifts in the 1916 front line. (This may be what A.N. Wilson was getting at - I haven't read much else about the first world war, but I find it difficult to believe that there are many other accounts of it that are as lucid as this.)
Of course, the effects of the Somme were devastating. Gilson and Smith were both killed, and Tolkien invalided home with trench fever; he never returned to the front line, fortunately. And it's fairly obvious that the deep friendship between Tolkien and Wiseman was fatally undermined by their war experiences. Garth makes a persuasive argument for the deep impact of the TCBS on Tolkien's writing. I would like to know more about the effect of Tolkien's relationship with his wife Edith, who he was courting and marrying at this time, on his writing. Perhaps there is little to say, or to be discovered.
There are two lengthy postscripts to the main narrative. The first looks at the relationship between what Tolkien was actually writing during the Great War and his eventually published work (two decades later for The Hobbit, four decades later for The Lord of the Rings). The second ranges freely across the whole spectrum of English literature in the twentieth century, pointing out that Tolkien describes both the heroism and the horror of war (where Owen and Sassoon concentrate on the horror, to the point of concealing what they themselves were up to), and concluding with a favourable review from C.S. Lewis about the realism of Tolkien's portrayal of the psychology of wartime.
There's lots more here. Recommended.