A Hobbit, a Wardrobe, and a Great War: How J.R.R. Tolkien and C.S. Lewis Rediscovered Faith, Friendship, and Heroism in the Cataclysm of 1914-1918

The publisher calls this "the untold story of how the First World War shaped the lives, faith, and writings of J. R. R. Tolkien and C. S. Lewis."