FEW WRITERS, past or present, have had careers quite as varied, dramatic, and contradictory as that of Robert Silverberg. Since he first became a full-time writer in the mid '50s, the world of publishing has undergone many radical changes, mostly for the worse. Writers have been called upon to serve the two masters of Art and Commerce as never before. Some have given their allegiance to one, some to the other; the vast majority have forged themselves one sort or another of private compromise. The number of choices has been almost as great as the number of writers struggling to survive. Yet Silverberg is still unique. He alone among modern American writers seems to have made all the choices at one time or another.

He is best known and best regarded for his work within science fiction. Yet he has left the genre twice-- once quietly in 1959, when the sf markets collapsed and Silverberg departed to write a distinguished series of nonfiction books on history, archeology, and popular science, and once very loudly in 1976, when he publicly announced his retirement because sf had no room for a serious writer, an announcement that brought on a storm of controversy. Even within sf, Silverberg has had at least four different identities. He has been a promising new writer, a hack churning out formula fiction, an ambitious writer at the artistic forefront of the field, and . . . what?

What indeed. Silverberg returned from his retirement in a big way with Lord Valentine's Castle, a huge and hugely successful sf adventure novel, but the Silverberg who returned was not the one who'd left. This fourth- stage Silverberg told a pretty good story, to be sure, but there was something more than a little mechanical about it. For all its success, Lord Valentine's Castle was a novel that went through the motions. It did not seem that Silverberg really believed what he was writing this time around the maypole. The book was curiously flat, even as an adventure, with none of the dark passion and pain that so infused Silverberg's best work, his landmark novels of the '60s and early '70s. It was a disappointing comeback for those who had admired the writer he had been. The disappointment became even more keenly felt when he followed Lord Valentine's Castle with a series of spinoffs and an announced sequel.

And now, just when everybody had safely reassessed Robert Silverberg once again and stuck him down in the "pleasant-entertainment-but-nothing-to-write-home-about" drawer, comes Lord of Darkness.

Silverberg has left science fiction once again, it seems, if only temporarily. Lord of Darkness is historical fiction, or perhaps more properly historical fantasy, the term the author himself applies to it in his afterword. But the labels don't matter. What matters is that this lord is as different from Lord Valentine as . . . well, as darkness from valentines. This book too is an adventure, yes. But this time Silverberg seems to care.

The novel is loosely based on the memoir of Andrew Battell, an English seaman of the Elizabethan era, although much of the detail and nearly all of the protagonist's character is of Silverberg's own devising. While privateering off the coast of Brazil, Battell is stranded and captured by the Portuguese, who ship him off to their African colony of Angola. There Battell spends 20 tumultuous years. For most of his stay, his only goal is a speedy return to England, but that prospect, though often close, continues to elude him while he plays a succession of other roles. He is by turns a slave and a slaveowner, a confidant of one Portuguese governor, the prisoner of a second, a common footsoldier for a third. He serves as a river pilot and master of a coastal trading vessel, loves a highborn Portuguese half-breed and a slave, participates in wars and exploration and local politics, and finally, betrayed one time too many, flees to the jungle. There he joins the dreaded cannibal Jaqqas and their chief, the Imbe Calandola, the lord of darkness; he wanders and wars in their company, shares their feasts, takes a Jaqqa wife, and ultimately becomes Andubatil, the white Jaqqa, before a final confrontation with the Portuguese.

Given its basic premise of a lone Englishman in an exotic historical setting, there is a certain temptation to dismiss Lord of Darkness as a kind of Shogun-in-blackface, but the book is really a good deal more than that. Silverberg is well versed in African history and myth; his best nonfiction work is his massive book on the mythical Afro-Asian kingdom of Prester John. He is also a devotee of Joseph Conrad; one of his finest sf novels, Downward to the Earth, was a retelling of Heart of Darkness in science fictional terms. These threads come together in Lord of Darkness. Perhaps Silverberg has lost faith in the imaginary worlds of sf, even his own Majipoor, but he believes in this material, and infuses it with the dark power that was so much a part of his best work, wedding a considerable amount of real pain to a compelling storyline, and producing a gripping and compulsively readable novel with more sheer page-turning narrative drive than anything he has done since 1976.

The book is also something of a minor stylistic triumph. Rather than telling the story third-person in modern prose, as Clavell did in Shogun, Silverberg opts for a harder path: Andrew Battell tells his story himself, in the first person, in his own voice. It's all sleight of hand, to be sure. If Silverberg had reproduced authentic Elizabethan prose, his readers would have left in droves. Instead he gives us a solid and highly readable novel that manages to capture just enough of the flavor of Elizabethan English, in its cadences and its vocabulary, to bring the whole thing off splendidly.

To be sure, the reader does catch an occasional glimpse of Silverberg peeking out from behind his character. Despite gradually coming to accept and even embrace cannibalism, for instance, the character is shocked and horrified when he hears of female clitoral circumcision, a juxtaposition of attitudes that seems more appropriate to a liberated writer from Oakland than to an Elizabethan seaman. The narrative is also marred somewhat by rather-too-liberal dollops of best- seller sex.

If these flaws are serious enough to keep Lord of Darkness from ranking with Dying Inside and Downward to the Earth and Silverberg's other first-rank work, they are still essentially quibbles, and do not lessen the impact of the book as a whole.

Robert Silverberg is back. This time, for real.