I started to read this book to understand what Michael D. O'Brien meant by "subcreation" in the prologue of his sequel to Fr. Elijah. And — which is stranger — why he used this concept to describe a work like Fr. Elijah, while Tolkien used it in a seemingly niche genre within the grander fantasy genre, that is, fairy tales. So, I discovered that Tolkien didn't intend to treat the matter exhaustively, nor explain or systematize it philosophically, but give us a philological analysis that only an erudite English professor like him could. Which also includes his unique experience as the "subcreator" of Middle Earth and a devout Catholic.
What he says about fairy tales isn't just about fairy tales. It goes for fiction as a whole.
"Subcreation" implies an order to Creation. In a sense, if a man can create another world in his head and share it with others, it's because of his nature and its properties of intellectual activity and imagination. God is at the beginning of everything, and if we can share truth through fiction, He wills us to do this. Our capacity to "subcreate" to communicate truth and good and beauty is far from a Promethean insurgency to create our idealized world purged from the flaws we see in the real one; on the contrary, God's still the First Cause of every work of fiction. It's interesting to think about this, for Tolkien seems to say that God's Grace, in redeeming men, seeks to redeem and elevate fiction as well.
This follows the fact that fiction ought to be a mirror of Creation, just as Creation is a mirror of God. St. Thomas said beauty is integral to being and truth and good, and requires harmony, integrity and luminosity. This is also the essence of fiction. But fairy tales and the fantasy genre as a whole are special, for they bring us back to our time of innocence, where hope shined brighter in our soul (Aristotle said the young are more vivacious for their hope and lack of experience) and we weren't yet hardened by the trials of adult life. Then, we see the benefits of consuming these works as adults: it helps us bridge the gaps between maturity and immaturity and integrate in our personality a part of the innocence we lost while tempering it with our acquired experiences. That's why Tolkien talks about Fantasy, Recovery, Escape and Consolation — it's a sort of journey that takes us to a deeper part of ourselves and the world around us and strengthens us to return to reality with a new perspective about things. It also propels us to desire to do good and flee sin.
The tale of "Leaf by Niggle" is a beautiful expression of everything I've just said. Niggle is a simple and good-hearted man, and the portrait of a good artist, since art is for the other, and it's just right to help people if you want to delight them with your art too — it's a matter of charity, not self-interest. He can't accomplish his goals and is misunderstood by his people, and is forced to travel and pass through trials and tribulations in a hospital, until freed by a tribunal. He learns to use his time and work at a consistent pace. He returns to his home and finds the Tree he was trying to paint — an idea come to life. He calls his neighbor Parish to polish the work and plant a garden and build a house, and then both part ways when a shepherd calls Niggle to the mountains. It's an allegory for going on after a work's end, to seek higher things, as every artist tries to surpass himself in every work. But we could interpret it as a metaphor for death after legacy, as the Shepherd, that is, Christ, calls Niggle to the Mountains of Paradise after his mission is done. And there's the fact that nobody recognizes him in his village, even after he finished the work of his life — for, as says Scripture, no prophet is accepted in his own country.
Being an artist is hard. Often, people can't recognize his worth or understand why he does what he does until the work is done. Then, they can see what he saw from the beginning and appreciate his journey, his hard work. Even so, there are artists the world will never know or recognize. And it's a solace to know that, even then, God will recognize them and, if they worked for Him, reward them in eternity.