One key to understand The Lord of the Rings and all the other great works by Tolkien is his involvement in the First World War. Elements like a last minute rescue of the Rohirrim, or the role of Samwise Gamgee, or the mechanical beasts entering the battle of Gondolin clearly refer to his experience in the Battle of the Somme.
Tolkien was there in the horrors of the trenches, as a Second Lieutenant of the 11th Lancashire Fusiliers, from June to October 1916. He survived because he caught trench fever, returned to England reporting sick and never returned to action.
Why doesn't the cover illustrate those trenches, then, why isn't there a tank, or soldiers in battle gear running through trenches? The photo is well-chosen, because it shows Tolkien as part of his beloved community at Exeter College. Consider, that Tolkien wasn't one of the first to be deployed as soldier, but decided to finish his degree in university first.
The core of this biography focuses on Tolkien's ways through the war. John Garth gives a fascinating portray, disentangles the complicated movements of WWI campaigns, and fleshes out how his schoolfriends of the T.C.B.S. club fared during the war. It is a highly involved and intense research into not easily accessible sources, and the author mastered them in a way which is accessible to a broader public.
One can literally watch the ideas leading to the Silmarillion coming to life. The author embeds and explains several poems from Tolkien and his friends through these early years. Tolkien started his mythology reluctantly before the Battle of the Somme. But only after he returned home, his ideas came to fruition in a kind of narrative explosion. His prose work started during his rehabilitation from trench fever back home in England, beginning with The Fall of Gondolin, continued with Beren and Luthien, and finished his Great Tales with The Children of Hurin. There was no idea of the Hobbit or the Lord of the Rings, and there was no Second or Third Age.
John Garth brings all this to life in a thorough amount of details. He contextualizes Tolkien as a war author. Where other authors of his generation like Graves, Sassoon, or Owen created a far more pessimistic, modern poetry, Tolkien reflected the fighting differently, staying with the naturalistic romances, taking a stance against the disenchantment of his time.
The last part of the book concentrates on the effects on Tolkien's later Middle Earth writings, how formative they were, and how his experiences influenced the world he created. Although Tolkien himself hated such interpretation, Garth's analysis makes sense to me.
A huge mass of literary references and notes are given at the end of the book. Garth's choice to not add footnote numbers in the text was a good one. The book finishes with twelve pages of bibliography and a handy index. More interesting to the casual reader will be the middle part with several photos of Tolkien, and his Exeter friends of the T.C.B.S, and the maps illustrating the movements of the Battle of the Somme.
You can see that this is not "yet another" Tolkien biography. It is a necessary one, adding much to the essential biography from Carpenter. The Mythopoeic Society honored the work with an Award for Inklings Study.
For further reading of the author, consider his Tolkien at Exeter College (review), and his newer Worlds of J.R.R. Tolkien: The Places that Inspired Middle-Earth (which I haven't read, yet).