IT HAS JUST been announced this month that Robert Holdstock's Mythago Wood (Arbor House, $14.95), first published last year in England, has won the World Fantasy Award for 1985. The novel, Holdstock's finest by far, is without doubt a fantasy, and has been given the right award for the kind of book it is. Just as certainly, and like the other books assembled here for review, it is also a novel of science fiction.

Twenty years ago, such genre-mixing would have been frowned upon in America, though not in Holdstock's native Britain. Here it was still thought that, in evolving out of fantasy, science fiction had in fact supplanted the earlier mode, and had valiantly substituted rational speculation about possible worlds for arbitrary spasms of escapist imagination about worlds that could never exist. Since 1965, there has of course been a significant failure of confidence in positivism within the genre. The brave new world has come, and we have not been saved. The family at the heart of Holdstock's novel goes by the name of Huxley, and may well represent an homage to the author of Ape and Essence (1948), a science fiction novel in which the horrors of man's intrinsic nature make mock of any scientific attempt to rationalize the cancerous irrationality of, say, Auschwitz.

Significantly, Mythago Wood is set at just about the time Ape and Essence was being written, and in its own way also mounts an attack upon the premises of enlightenment. Steven Huxley has served in the war, and returns to his home in rural England upon the death of his father, who had spent his last years in pseudo-scientific attempts to penetrate the mysteries of Ryhope Wood, a three-mile-square patch of primal forest adjoining the family house. As Steven soon learns, strange creatures inhabit Ryhope, figures like Robin Hood and Arthur and others far more ancient. These mythagoes (the term is a comical "scientific" combination of myth and imago) represent a marriage of the deep wood's chthonic power to the unconscious creative shaping urges of the human race, or so Steven's father had speculated. But Ryhope itself resists entry.

That it also resists explanation goes far to explain the insinuating appeal of Mythago Wood. Steven, like his father and older brother before him, falls in love with a mythago named Guiwenneth, a primal figure of whom Guinevere is a mere avatar, and his speculative curiosity about Ryhope becomes a transforming obsession. Literally so. Like the other books in review, this novel is a tale of metamorphosis. (It might be suggested that genre-mixing and metamorphosis go together quite naturally: it is certainly the case that to switch a tale from sf to fantasy involves a radical shift in perspective on the characters in that tale.) The only way to penetrate Ryhope, Steven discovers, and so to find Guiwenneth again after she has been abducted by fellow mythagoes, is to inhabit it fully.

He must become a figure of myth himself. His search for Guiwenneth must give off an archetypal glow, just as his conflict with father and brother, both of whom have preceded him into Ryhope, must somehow re-enact the primal family drama. The more fully he enters into Ryhope, the bigger Ryhope becomes; in the heart of the wood an entire world, ancient and raw, unfolds. (The resemblance here to John Crowley's magisterial Little, Big may not be accidental.) Sf and 1948 are left very far behind indeed. At novel's close, Steven stares at us out of the depths of his metamorphosis like a deep imago of our most private dreams. Paul Hazel

THERE IS no quick way to appreciate Winterking (Atlantic Monthly Press, $18.95), the remarkable climax to Paul Hazel's "Finnb' trilogy. All three volumes must be read. Yearwood (1980) and Undersea (1982) are both fantasy novels, closely linked, told with an unvariant severity of purpose: tense and relentless versions of Welsh mythology, they tell of the adolescence and assumption of kingship of Finn, child of a selkie and a witch. The books amount to a ravening dance of metamorphosis as Finn treads the dark path to his final kingship as Duinn, the lord of death from under the sea. To one lad, Wyck, almost by chance, he gives free passage across the river into his kingdom. Wyck, therefore, cannot die. He cannot even age.

Winterking is set in the 20th century, in a New England strangely different from our own. There has been no American Revolution. Young Will Wyckham, heir to a huge fortune, is able, on a whim, to make his banker a life peer. The world Wyckham (or Wyck) inhabits is autumnal, idyllic, reticent, full of polished detail. It is a pastoral America, a 20th century without horrors or stigmata. But it is threatened.

Winter approaches, and this alternate universe (it is a common sf device) trembles at the root. Wyckham retreats to his great nine-cornered house at the center of things (another echo of Crowley's magnum opus) and behind a thicket of thorns awaits the moment of Duinn's coming, when all things must change. But somehow there is a kind of survival. In the morning, after the great wind and the rain of portents, Wyckham and his chosen companions look into a wilderness of snow and prepare to inhabit a new world. It may be ours. Clearly, much of Winterking's strength and nuance comes by contrast to its predecessors in the trilogy; and certainly it is incomprehensible without the earlier books. Wyckham's immortality would be a cheap ploy out of the ragbag of sf and fantasy tricks, if it were not justified by its arduous context. Within that context, Winterking is like a blessing, a garden of delight. It should be read, and reread. Robert Heinlein

AND WHAM-BANG into the infinite universes of the latter-day Robert A. Heinlein, where anything can change into anything, and nothing means much at all, not death, not love. But The Cat Who Walks Through Walls: A Comedy of Manners (Putnam, $17.95) does start off at a great clip, and looks for a while as though it's going to mean something. Unknown agents are trying to kill Colonel Ames, first on a space habitat, then on the free-enterprise Moon of The Moon is a Harsh Mistress (1966), a 100 years after the close of that book, Heinlein's last real sf novel, in which he coined the libertarian slogan, There Ain't No Such Thing As A Free Lunch. Well, TANSTAAFL ain't what it used to be. The Moon has become a dreary, predatory dystopia. Even the Colonel -- a typical Heinleinesque Competent Man -- is finding his fellow libertarians a mite distasteful.

But just as we're getting to grips with a real issue, Heinlein transforms his sf tale into one of his fantasies of infantile omnipotence. The Colonel's wife turns out to be one of Lazarus (Time Enough for Love) Long's innumerable and indistinguishable pals, and the Colonel's universe just one of a laxative infinity of universes. There is instant time travel to any point in any universe. Everything is redeemable. Long and his pals are engaged in policing everything, seemingly for the joy of it. The story dies -- though clearly there is going to be a sequel. The secret of Heinlein's late works is that TANSTAAFL only applies to us plebes. For Lazarus Long and the Chosen Few, the lunch is free. It leaves an odd taste. Gene Wolfe

IF THERE IS a free lunch at the end of Gene Wolfe's superlative new comedy of metamorphosis and redemption, then it has been earned. In undertaking first to protect and then to find old Ben Free, the four protagonists of Free Live Free (Tor, $16.95) must first struggle through the comic inferno of a subtly transformed world, and must then gain their hearts' desires only to find them wanting. Only at the end of their adventures will an unmasked Ben Free show them that redemption -- true happiness -- the real world lies within them, and has in a sense done so all along. He will then send them back through time to redeem their earlier selves by living their own lives anew and transfigured, just as Severian does in Wolfe's great Book of the New Sun.

From almost the first page of Free Live Free, Wolfe makes it clear that his fable can be read as a loose fantasia on the 1939 film version of The Wizard of Oz. When the four finally corner Free at the controls of the vast balsa-wood propeller-driven airplane and time machine through which he rules the world below, the connection is both pointed and deeply comical. But Oz is by no means the end of it. Free Live Free reads as an attempt to recapture the essence of the world of the American film before the War. Not only Oz but The Maltese Falcon; not only the young Howard Hawks but, more importantly, Frank Capra. In its lucid nostalgia, and in its magic-realist populism, Free Live Free is very much Oz as Frank Capra might have dreamed it.

Even the limited edition of the book, published last year by Mark V. Ziesing at $45, was so clear and compulsive in the telling that it could be read at one long go. Slightly sharpened at points, and shorn of the dithery Chapter 43 of the original version, Free Live Free now flows with an unbroken liquid precision from the next world to this, from fantasy to sf, from dream to a magically- achieved reality. Like an enormously expanded version of the tales-within-tales that punctuate Wolfe's earlier novels, Free Live Free is an exemplary fable. It is humorous, loving, loyal and true. From an author whose sensibility is normally complex and serenely chilling, it reads like a present from the heart. It is blessed to give.