Here is a list of readings that I’ve found related to the creatures that Lewis calls the longaevi (long-livers). Pease comment with other readings that you have found. I would be glad to collect more. Meanwhile, enjoy some of these:
- C.S. Lewis lecture on “THE LONGAEVI” published as chapter VI in The Discarded Image (see some excerpts here).
- J.R.R. Tolkien essay “On Fairy-Stories” (see extended excerpts here).
- Roland in Moonlight by David Bentley Hart
- David Bentley Hart essay “The Secret Commonwealth.”
- David Bentley Hart essay “God, Gods and Fairies.”
- David Bentley Hart essay “Therapeutic Superstition.”
- David Bentley Hart essay “…Of Hills, Brooks, Standing Lakes and Groves…”
- David Bentley Hart book review “Selkies and Nixies: The Penguin Book of Mermaids” from The Lamp: A Catholic Journal of Literature, Science, the Fine Arts, Etc. Issue 2. Assumption 2020. pp. 49-50.
John Milbank lecture “Stanton Lecture 8: The Surprise of the Imagined.”Stephen R.L Clark essay “How to Believe in Fairies” from Inquiry: An Interdisciplinary Journal of Philosophy 30 (4):337 – 355 (1987). Read out loud on YouTube here.Stephen R.L Clark essay “Why We Believe in Fairies.”Robert Kirk booklet The Secret Commonwealth (1691).Henry Corbin essay Mundus Imaginalis or the Imaginary and the ImaginalSeyyed Hossein Nasr essay “In the Beginning was Consciousness” that appears to be chapter 14 from The Essential Sophia (Library of Perennial Philosophy, 2006) by Seyyed Hossein Nasr and Katherine O'Brien, a book that is not in print as of this posting (November of 2020).Joshua Gibbs essay “Will Fictional Characters Be In Heaven?”Foolish essay by yours truly called “Building Folklore Wealth” (almost on topic in this list).
Copious Flowers 
Hey, nice list! I have recently discovered the wonderful writing of Robert Ogilvie Crombie on his encounters with fairies, especially Meeting Fairies: My Remarkable Encounters with Nature Spirits, and Encounters with Nature Spirits: Co-creating with the Elemental Kingdom.