Randel Helms’ book is an old work by now, but for all that I thought it is still good. His concern is to figure out what Tolkien thought he was trying to achieve. What is the point of fairy-tales? How do you have to build a secondary world? How did these answers arise and then get answered by Tolkien? That sort of thing.
The start of the book can seem wobbly, a bit pedantic, and especially the third chapter in which he uses the reproductive metaphors of psychoanalysis (aware that Tolkien would greatly object), but he has a point, and it gets made. When you translate The Hobbit into that system of obvious metaphors, you see a consistent meaning. He uses this to convince his perhaps more skeptical readers that the whole of his enterprise is not in vain.
I think his argument can hold. He has modestly refused to believe his take on The Hobbit as an incomplete attempt at what the trilogy later succeeded in doing is the only possible one; but as a grown reader who still loves The Hobbit, I think he makes sense out of Tolkien’s later dissatisfaction with the book, and it had puzzled me. Helms’ journey through Monsters & Critics and On Fairy-Stories, and then in a later chapter through Leaf by Niggle and Smith of Wooton Major are both instructive and compelling.
Helms did a lot with limited resources, since he did his work and handed in the manuscript right as Tolkien died and before even The Silmarillion was published. But if you want a good theoretical view at the thought behind the Lord of the Rings, starting here would work.