DAVID BRIN's new book is nothing if not ambitious. The Hugo and Nebula Award-winning author of Startide Rising, The Postman and The Uplift War has written a massive sf novel whose main concern is the very future of our planet, both ecological and cultural. Earth (Bantam Spectra, $19.95) is about equal parts a thriller -- featuring a long list of characters -- concerning a runaway black hole at the Earth's core, and a preachy political tract out of the Jonathan Schell school.

Brin's heart is in the right place, but his scattershot approach is often distracting. His dialogue of 50 years hence features such futuristic profanities as "dumpit," hardly a startling or convincing neologism. In narrative passages he is prone to overstatement. And he sometimes employs as many as four or five eyestraining fonts on a single page, with lots of italics, exclamation points, and boldface -- as well as charts, diagrams, news clippings, computer printouts, etc.

The latter elements are not new, of course. John Dos Passos did this sort of thing some 70 years ago in his famous trilogy, USA. It was a dynamic literary innovation at the time, and there was a similar freshness to John Brunner's use of the same effects in his groundbreaking sf novel Stand on Zanzibar in the late '60s. By 1990, however, the fictional collage seems a shopworn gimmick at best. Perhaps Brin is not confident that his readers will grasp his grand theme, and has used this paste-up technique as a way of drawing attention to its global significance.

In spite of this typographic busyness and the lack of fully fleshed-out characters, the pages keep turning as scientists race to discover a way to prevent the black hole from destroying our planet. There is a number of international and outer space locales that are interesting and reasonably well handled, too. The novel's sincerity grows on you as it hurtles along, its wealth of detail is quite impressive, and the ending is reasonably satisfying for such a lengthy book. With the environmental movement's resurgence currently in the news, Earth might very well become a trendy best seller. A Case of Conscience WE MOVE far away from Earth in Michael Berlyn's The Eternal Enemy (Morrow, $19.95). The setting is a planet called Gandji. There, humans are at war with a race of aliens called the Habers. These creatures are able to combine their genetic material with any other living being at will and create Haber hybrids, using as technology only . . . uh, crystals.

This is New Age nonsense, of course, and the mechanics of such an unlikely process is dismissed in a single, short paragraph attributing this biological miracle to a "byproduct of a virus." A byproduct? Why not the virus itself? Watson and Crick are already out of the picture, so why back away from what is clearly fantasy in science fiction drag? Berlyn's writing lacks self-assurance throughout, offering such terminology as "furlike" instead of furry, and "osmoticlike" for osmotic. An alien landscape is described as merely "strange-looking."

Anyway, the Habers are at war with a race of giant insects, who descend like -- honest, I'm not kidding -- a plague of locusts. Human genetic material is needed to fight the bugs, because Habers just aren't mean enough. Apparently, this is intended as irony -- the nasty human crew is humanized by aliens -- but reads as the most transparent of contrivances. All of the human characters are tediously monstrous, without a single redeeming virtue. Even their venality is torpid, exemplifying all too perfectly Hannah Arendt's "banality of evil." These people simply want to kill aliens; when they're not slaughtering the native population, they relax by getting into fistfights with one another.

It would seem that "NASA 2" had to search pretty far afield to man a crew with so many violent misfits. Only our hero, Markos Dressler, has a kind word for the Habers, and he's biased, what with his genetic transformation by the godlike beings of Gandji having occurred in the book's opening chapters.

When a science fiction novel's science is ludicrous, its characters cardboard, its narrative sluggish and its prose clunky, one can only hope that the dialogue will have some zing to it. The following exchange between Markos and "Alpha, the Old One," clearly intended as a pithy response to the aberrant behavior of the humans, is a sampling.

" 'Yes, Old One, you're right. Now we must do what we can. We must let the Terrans die.'

'Die?'

'Yes. Dead humans don't argue.' "

Nor do they wear plaid. Crenelated Visions GENE WOLFE might dispute Markos's sage advice, judging from Castleview (Tor, $18.95) a new fantasy novel by the Nebula Award-winning author. Here, not only do the dead come back to life, but a small midwestern community is visited by the heroes and villains of Arthurian romance. If you feel that there's nothing more to be done with the Camelot myth, you'll be pleasantly surprised by this fast-paced, engaging, even endearing book. Castleview is peopled with finely drawn characters, such as automobile salesman Will E. Shields, his cookbook-writing wife Ann Schindler, and their charming teen-age daughter Mercedes Schindler-Shields.

Staying at a motel in the town of Castleview, the Schindler-Shieldses are subject to shootings, a giant on horseback, trolls, Morgan Le Fay and a castle that vanishes and reappears magically; this last is the persistent optical illusion for which the Illinois hamlet was named. These adventures have an oneiric power that is carried along by the author's often poetic language.

This novel is far more accessible than such recent efforts as Soldier of Arete, though it may lack that novel's ambition. Whatever the scope of his aspirations, Wolfe's current meditation on the magic lying just beyond the veneer of ordinary life is heady and beautiful stuff. The theme of sacrifice emerges from seemingly disparate strands of narrative and is delicately developed within the constraints of the novel's dream logic. Castleview is a thorough, innovative and craftsmanlike job applied with a real artist's singular and sure hand. Terrible Lizards DIVERS HANDS are at work in Dinosaurs! (Ace, $3.50), the latest in a long series of anthologies edited by Jack Dann and Gardner Dozois, who also contribute a cute, collaborative vignette, "A Change in the Weather." Other stories range chronologically from the 1956 L. Sprague de Camp classic, "A Gun for Dinosaur," to the introspective 1987 Steve Rasnic Tem story, "Dinosaur."

Brian Aldiss's often reprinted "Poor Little Warrior" still packs an ironic wallop, and "Dinosaurs" by recent Nebula Award-winner Geoffrey Landis takes an unexpected turn into the Mesozoic. Arthur C. Clarke contributes a neat puzzle story, "Time's Arrow," in which an extremely rare fossil is unearthed by a group of British scientists. American Indians and dinosaurs figure in Howard Waldrop's intriguing "Green Brother," while cowboys and dinosaurs have at it in Sharon N. Farber's humorous "The Last Thunder Horse West of the Mississippi." A prehistoric beast other than a dinosaur is the ghostly denizen of Edward Bryant's "Strata," rendered in prose so evocative that the reader doesn't mind that an actual dinosaur never lumbers on stage. The entire delightful package is wrapped in Bob Walter's lovably lurid cover painting. What more could a dinosaur fan ask for? Tim Sullivan's latest novel is "The Parasite War." He is completing "The Martian Viking" and editing an original horror anthology, "Cold Shocks."