Tolkien and the Great War: The Threshold of Middle Earth by John Garth
I had this book out from the library, and I have since returned it, so this is from memory.
Tolkien denied that the Lord of the Rings was any sort of allegory for real-world events (at the time he was asked, the event in question was World War Two). Yet it's difficult to deny that his experiences in the Great War influenced the book at all: the desolate wasteland of Mordor as the no-man's-land between the trenches, shell-scarred, muddy and barren; Frodo's return to the Shire, unable to regain the happiness he'd had there once.
Garth begins with a brief summary of Tolkien's childhood, picking up in greater detail around the time he was about twelve and met his best friends, whose small in-group would inspire him to create Middle Earth, and which was cut in half by the war.
He interweaves history, via letters and battle movements, literary criticism, and the development of Middle Earth, from its conception as the House of Lost Dreaming to its eventual final form.
From the perspective of someone who read LOTR in elementary school and had copies of the Book of Lost Tales and the Silmarillion in high school, this book was very interesting. I learned a lot about the beginnings of Middle Earth, and I thought it interesting to see hints of the things I knew.
It is also, in parts, a good look at life in the trenches during the Great War, with the senseless death and general futility. More general histories of wars tend to overlook the personal details, listing casualties as numbers; this makes no few of those statistics into real human tragedies.
If you are a Tolkien fan, I would recommend this book.