HELLICONIA WINTER by Brian W. Aldiss (Atheneum: $17.95). Brian Aldiss brings his trilogy on the life and times of the planet Helliconia to a close. Helliconia's bio-sociology is run by its double suns and extremes of weather. The extremes also create changes in the planet's "human" population. A serious, literate writer, Aldiss takes the people of Helliconia into the twilight of their world as they know it: 600 years of winter face them now. Holding on to the slippery sands of civilization against the possible slide into barbarism is the force driving the plot. Luterin Shokerandit is the hero of this tale; he goes off to fight an ill-fated war, and, hoodwinked by his superiors, discovers that he can never return home to his estate in the North and is transformed into a fugitive. He comes down with (but survives) the dreaded Fat Death, a plague transmitted by one of the other sentient life forms of the planet, the phagors, which creates a needed biological adjustment for the human race so it can survive the oncoming epochal winter. Meanwhile, an Earth observation station, the Avernus, has circled the planet for many hundreds of years recording the struggle of the life below and broadcasting it back to Earth as entertainment. Granted this subplot gives the reader a point of reference, perhaps, but in this book, it has no impact on the life below and becomes only a mildly interesting addendum. The tale is similar in scope to the other two books in the trilogy: fine characters, exotica and the grandeur of the landscape. But overall, it's only semicompelling as the end is given, because the reader knows the history of the planet (else why read the third book before the other two). There are no real twists, and an unrelieved seriousness permeates the novel.