I should make a caveat before starting into this, but John Bellairs is one of my favorite young adult authors. However, though I had read numerous middle grade titles I had never read Face in the Frost. And if you were to have asked me back in the day if I thought Bellairs was a "children's author" I would have equivocated. His writing appeals to some rather "adult" topics, and though his protagonists are decidedly middle graders, their demeanor and personalities were very mature for their age. This novel is definitely written for adults. Middle graders could of course tackle it, but the themes here, the characterization, and the scenery and settings are written to a rare intellect. And to top it all off, this novel was included by Gary as a seminal work of the D&D oeuvre.
In Face in the Frost we have on display some of the same fantastic-real world setting that is common to fantasies of this period. Indeed, drawing on historical figures such as Roger Bacon, and Prospero (though no the one you're thinking of). Starting rather comfortably in a cozy, if weird, Bellairsian home the story sets off with some "odd goings on" in the wizarding world that hints at potentially dark forces at work. And weird here expands to encompass not only the home, but to the whole world and adventure these wizards set off upon. Shrinking spells, crystal balls, illusions galore and magic books are just some of the classic eldritch tropes so well known in D&D.
But what this book grants, perhaps with the exception of Vance's Dying Earth series, is the flavor of D&D wizarding more than any other. The idea of a wizards "sanctum", the bizarre nature of magic itself, the almost comical reality that magic creates, and the literally no-holds barred possibilities that such magic generates are perfectly captured by the magical adventure of Face in the Frost.
I think this is noteworthy. Gary Gygax is known for not being rather fond of magic users. Whether this is due to Howard's, and his scriptoral kin's, tendency to write spell workers as the bad guys; or because he felt a personal love for mighty fighting heroes is up for debate. Others have said it was due to the magic-users potential to become too powerful, and Gary was always looking for opportunities to clip their wings. Even Tim Kask has made the point that he had to argue for giving magic-users an unerring strike via magic missile, saying "give the poor magic-user something!" Because Gary wanted to make them have to roll to hit with their one effective damage causing spell at low levels.
But I don't think Gary "hated" magic-users. Maybe I'm wrong, and I certainly wouldn't want to take umbrage with someone as wise and experienced as Tim Kask. But you have, according to Gary, in works like this one, and others, the clear idea of what D&D wizardry could be. Mysterious, weird, comical, yes, but also very, very powerful. If you're looking for potential archetypes while playing a magic-user you need look no further than works like this and others in Appendix N. Face also provides for would be DMs, above and beyond other works, what an adventure might look like for high level magic-users. With an opponent and the associated weird setting to match their skills and abilities. Gary was fond of constructing such adventures, which in my mind bring to memory White Plume Mountain, Tomb of Horrors, and his later Dungeonland and The Land Beyond the Magic Mirror.
Finally, what some OSR games have tried to regulate, namely the weird and dangerous element of magic in old school fantasy is already there in the spirit of AD&D as well as the rules themselves. The DMG held tables for insanity, potion miscibility, magic item destruction and malfunction. And nothing short of the DMs imagination inspired by works like Face in the Frost can sculpt the warp and weft of a world where magic is very much indeed a strange and unusual power that few dare lay hold of. What has inspired later OSR developers has been that few actual rules enforce the weirdness or danger of magic. Gary left this to DM and player imagination. And for those looking for the heart of it, Face in the Frost delivers in spades.
I might also pay homage to the creation of sinister, sorcerous villains in Melchior. Able to plane shift his enemies, wrap the world in perpetual winter, and create and place all sorts of magical obstacles in the way of our protagonist Prospero and his erstwhile companion Roger. It takes more than a simple bag of hit points to challenge high level wizards, and Melchior is the kind of villain such players need to challenge their mettle and thrill their adventuring chops. And, moreover, we have the wizard's guild, though this idea of wizarding brotherhoods is at home in several works in Appendix N, that come to his rescue near the end. This too is a cool idea to use when crafting high level challenges for magic-users. One would want to be careful of too much deus ex machina with such a device, but used strategically it has much potential.
I hope my love for this novel has not been too much on display, but not since my reading of The Fourth Dimension by Rudy Rucker has my mind felt like it was under the influence of actual magic. I can not recommend it highly enough. It is one of those books, however, that I'm protective of. I dare not share it with those who might not appreciate it's fine vintage. I so wish Bellairs had written more fantasy like this. Ah well, Lewis Barnavelt is a fine companion to spend time with.