A prequel to The Face in the Frost is a short story written by John Bellairs in the early 1970s for inclusion in a book written by author Lin Carter. Carter's book was never published and Bellairs's story is considered lost.
BackgroundImaginary Worlds: the Art of Fantasy, is a study of the modern literary fantasy genre written by noted author Lin Carter (1930-88) and published in 1973. It is the fifty-eighth volume in the celebrated Ballantine Adult Fantasy series. This entry is the “earliest full-length critical works devoted to fantasy writers and the history of fantasy” and Carter’s third study of the genre, proceeded by Tolkien: A Look Behind "The Lord of the Rings" (1969) and Lovecraft: A Look Behind the "Cthulhu Mythos" (1972).
Chapter eight, entitled "The Young Magicians: Some Modern Masters of Fantasy", is of particular interest because it discusses Carter’s three choices as the best fantasy novels to appear since The Lord of the Rings. His selections are The Last Unicorn (1968) by Peter S. Beagle, Red Moon and Black Mountain (1970) by Joy Chant, and The Face in the Frost (1969). Carter includes the following passage about Bellairs at the tail-end of his analysis of Face:
A graduate of Notre Dame and the University of Chicago, he is a teacher-turned-novelist who now lives in rural Massachusetts, where he is busily concocting further adventures of Prospero and Roger Bacon. I wrote to him after reading The Face in the Frost, and we have been exchanging letters off and on for some time now. An affable chap, he has let me look at his sketchy maps of the South Kingdom and some unpublished scraps, notes, and outlines for these further adventures; and, in fact, he has produced for my yet-unpublished anthology of juvenile fantasy, entitled Magic Kingdoms, a new short-story which tells how his diabolic duo first became friends. I confess myself hopelessly smitten with the South Kingdom, with Prospero, with his house (which sounds like the sort of place I'd like to live in myself), and very much infatuated with his jolly, mad mirror---even if it does show nothing else but the eighth inning of the 1943 Cubs-Giants game.While Carter hints Bellairs's short story was completed, Magic Kingdoms was never published.