Reddit mentions of Tolkien: Man and Myth: A Literary Life

A great book to read about the LotR and Tolkien is by Joseph Pierce, "Tolkien: Man and Myth"

In it is discussed the importance of man vs world alone without aid of huge magics or elves or fancy things. Just man's will, duty, perseverance, and sacrifice: and this Sam had in spades. As the hobbits represent the common man, Sam is the quintessential and epitome of hobbits (common men). And it is the inner strength found inside him that truly defeats the evil at the last hour.

He disliked the word allegory. That doesn't mean the story isn't. It's an allegory of Catholic teachings, from original sin to the three roles of Jesus. I'm on mobile too, but one source I can pull from, Tolkien: Man and Myth, which contains a number of his letters discussing the Catholic themes and imagery of the books, ( http://www.amazon.com/Tolkien-Man-Myth-Literary-Life/dp/0898708257/ref=sr_1_1?s=books&ie=UTF8&qid=1407207389&sr=1-1&keywords=Tolkien+man+myth).
Quick answer, he didn't like LotT viewed as a straight allegory because allegory is an extended metaphor, but LotR only draws from certain aspects at certain times. For example, Frodo, Gandlad and Aragorn each fulfill one of the roles of Jesus as priest prophet and king respectively, but only Gandalf dies and resurrects.
Edit: also, he basically argues against whatever people wanted to think he meant (WW2, the One Ring as nuclear power or the atomic bomb, etc.) and says in other letters he only drew from his Catholic worldview, and that was the only deeper meaning he intended. I'll pull those letters together tomorrow, they're actually a pretty fascinating look into his view on Middle Earth