This is still about Lewis, but does not fit well enough in the Undeceptions series.
David Foster reminded me that he had an extensive post over at Chicago Boyz discussing the book back in 2014, which I had commented on then. It saves me a lot of trouble to refer you to that, and he brings up points I would have neglected. I also discussed it in my recent post Surfeit of Creativity. Once you have weathered those two you may not want much more, but I do have a bit to add.
There is a type of fundamentalist who rejects Lewis because of his use of magic in his books, which I discussed incidentally a few weeks ago in Post 7400. They subscribe to a late Medieval view of witchcraft which they declare to be scriptural. However, in the case of this book, I think they get it right. In his seven plots, with too much going on in general there is an Arthurian plot which culminates in Merlin, an ancient pagan whose conversion to Christianity did not erase his previous wizardly abilities, being "used" by - possessed, really - pagan and subchristian forces to defeat the ancient evil in a wood at an English university. Lewis tries to stuff this under a common lesson of his that all things which die and submit to Christ can be resurrected and made holy, but there's one difficulty: they haven't died and submitted so much as lingered and been forgotten. It is an unnecessary ending. A more clearly Christian power defeating the demonic forces, some version of Archangel Michael could have been developed instead. Also, Jane Studdock's clairvoyant powers are clearly an inherited ability, not a spiritual gift in our usual understanding. Tolkien felt that the influence of Charles Williams brought this about, as his books contain these pagan and magical elements more commonly. I have not read him. I am told that he keeps those powers under control and under submission, so that Christians might read them with profit. J I. Packer (who died in British Columbia this summer at 93) was a fan, for example. Maybe so. Perhaps I am too fussy or too skittish on the matter because of my own Jesus people heritage in the 70s. But I think Lewis got it wrong here.
One more thing: There was a book by Peter Kreeft, the Catholic convert who taught for many years at Boston College, Between Heaven and Hell: A Dialog Somewhere Beyond Death with John F. Kennedy, C.S. Lewis, and Aldous Huxley. Kennedy, Lewis, and Huxley all died on November 22, 1963, and Kreeft imagines a conversation among them in those first few hours while awaiting...he doesn't say. It is a short book and a fun read. Kennedy is the humanist, secularised Christian; Lewis the orthodox believer, the Mere Christian; Huxley the orientalised Christian that was becoming every more common among the intellectual classes. Their alliances in the discussion shift, with Huxley and Lewis teaming up to argue at Kennedy at some points, Kennedy and Huxley allying to refute Lewis.