Happy anniversary and all that, but Victoria Holt, celebrating her 20th year of writing romantic-suspense best-sellers, emerges this time around, not with banners waving but with energy flagging. This will not discourage her fans (we are loyal beyond such little setbacks), but others, who would rather enliven their time than kill it, may prefer to pass up this one.

The illegitimate daughter of a man ho was unfortunately obliged to kill his brother, little Suewellyn Mateland flees with her parents to a volcanic South Sea island and grows up in several uninteresting ways.

The reader has the feeling that any minute now the main story will start, but opportunity after opportunity falls prosaically in Suewellyn's wake, drowned under one and two-syllable words and an unending succession of short, declarative sentences.

Striking a blow for topicality, Grumbling Giant, the volcano, erupts at last to wipe out certain awkward family connections, but even this falls between the chapters, and we are treated only to a couple of perfunctory des-criptive paragraphs in the dialogue.

The shrift is just as short in regard to Mateland Castle, touted as fascinating enough to inspire murder; its interior is described only by the room: "There was the drawing room, the dinning room. . . the library, the armory, the gunroom," reminding one of the old Vaughan Meader routine about Jackie Kennedy's TV tour of White House paintings.

Eventually Suewellyn is set up for the big adventure, which involves impersonating her half-sister as heir to the castle. Over a period of months, this plot line picks up a few exciting moments, but not many.

The biggest mystery in the book is why the dust jacket artist ignored the pivotal mole near Suewellyn's mouth, and the next biggest mystery is whatever happened to the flaming passion promised in the jacket copy. Only intermittently do the sweethearts sound enthusiastic about each other, and at the very end it takes the hero and unexplained three months to get around to rescuing Suewellyn from loveless oblivion. Gothic lovers should burn with a clear, pure flame. They are not supposed to sputter.

Hotter at the core and far more provocative, "The Janus Imperative" features German journalist Max Steiner's search for the significance of the word "Janus," whispered to him by two men shot to death in his presence 25 years apart. Naturally, he enlists the services of one of the victims' widows, lovely and elusive Minna Walther, and of course they fall in love. Luckily, his extant wife has turned maternal on him, and his two children are mean-type kids, so we needn't worry to much about them.

Anthony, author of 10 books of fiction and 11 of history, uses an unimaginative style, but her structure is enveloping, binding the reader's interest tighter as the details unfold. Identifying Janus is not as easy as it seems at first, and the author skillfully paves the way for several wrong guesses before the startling truth is revealed.

Minor characterizations are fairly strong, in several cases. Two of the most fascinating are a homosexual couple who might be rather endearing in their commitment to each other and their preoccupation with the scenery, retirement plans and what the Church is coming to, if it weren't for their livelihood, which is political assassination. Even more amoral, and straight out of the Gordon Liddy Textbook, is the American intelligence operative, a distressing embodiment of all the CIA stereotypes held throughout the world.

But this is not a tale, like many which have emerged in recent years, that blurs the edges and muddies the debate of good and bad with the confusions of reality. The evil is very evident here, and the victory over it is clear-cut. r

There are several things one might disapprove of in "Janus," like the genetic theory of evil, for instance, and the violence of the climax. Still, it's exciting, engrossing and altogether deserving of languorous indulgence on a hot day in the hammock.