Helliconia is a massive tripartite work about which extensive, rigorous scholarly essays have been written. This is not going to be such an essay.
Eight million years or so ago, the sol-type star Batalix was captured by the much younger and brighter supergiant Freyr. Since then, the inhabitants of Helliconia, Batalix's sole life-bearing planet, have endured a short year of 480 days as their world orbits Batalix and a great year of 1,825 years (about 2,500 terrestrial years) as Batalix, planetary retinue in tow, orbits Freyr. Over the great year the climate varies hugely, from the centuries-long winter, which is almost too cold for the survival of Helliconia's humanoid population but ideal for its population of phagors, the other dominant intelligent lifeform, to the centuries-long summer, which is almost too hot for the survival of humans and phagors alike.
An important additional element of the environment is the helico (sic with one "l") virus, which is spread by the ticks that habitually cover the phagors but affects humans. It's active just twice a year, in the spring and fall, when it manifests as, respectively, bone fever and the fat death. Both diseases infect almost the entirety of the human population, with perhaps a 50% mortality rate. Survivors of bone fever emerge in slimmed-down form, and thus better able to survive the summer; survivors of the fat death emerge, after a period of obsessive eating that may even involve cannibalism, with vastly increased body mass, and thus better able to survive the rigors of winter.
Also of note is that a satellite built by people from earth, Avernus, orbits Helliconia, observing all the surface goings-on and beaming the images back home to earth in a continuous stream.
Volume 1, Helliconia Spring, is really a (very) protracted exercise in world-building, as we're introduced to the workings of Helliconia and a staggering amount of its flora and fauna. There are bits and pieces of coherent story thrown into the mix, especially in the first hundred or more pages, but nothing that one might really regard as an overall story arc; they all seem somehow inconsequential, petering out when their usefulness is done rather than coming to a proper conclusion. Although I'm sure this wasn't Aldiss's intention, the stories and story fragments feel as if they've been tossed in merely to keep us reading through the long expository passages.
Add in that the text is frequently marred by effusions of unwisely pretentious vocabulary ("Alehaw arranged himself with one befurred elbow on the rock, so that he could tuck his thumb deep into the hollow of his left cheek, propping the weight of his skull on his zygomatic bone" -- and precisely which bloody bone is that one, prithee?), and it's not surprising that, at the end of the very long Volume 1, I looked at the oncoming, even longer Volume 2 with something less than a song of glee in my heart.
Helliconia Summer was, however, a far easier and more enjoyable read. There are still eruptions of daftly obscure vocabulary and tracts of exposition, but they're all held together by a multi-stranded plot that sustains the interest -- a plot that, while its events are changing the course of Helliconian-human history, is full of incidents at the smaller, individual scale. One strand of this plot is that the tribes of the Avernus have begun a lottery whose prize is that the winner is sent down to the planet's surface. There s/he will inevitably die fairly soon, if not by hostile action then by infection with the helico virus, which is always fatal to terrestrials, but in the interim will at least have the chance to live. This time the winner is young Billy, and he does indeed do some livin' even as he's caught up in the periphery of the political/religious drama that's molding the fates of millions.
Helliconia Winter, shorter than the other two, sees the world speeding back toward icy conditions once more. The phagors, who've spent much of the summer in uneasy coexistence with the humans, or more often in servitude to them, are beginning to flex their muscles once more, the fat death is spreading, and a completely ruthless oligarchy is planning the survival of human civilization through the winter by destroying all dissidence and exterminating the phagors. We suspect this scheme is going to fail, just like every other previous pre-winter scheme, and that next spring is going to be very much like the one we've already seen . . .
It's easy enough to describe Helliconia as one of science fiction's great triumphs of world-building, because that's what everybody else has done, but I have my doubts. While there are some interestingly alien components of Helliconia's flora and fauna -- the phagors themselves might just about qualify, with their odd articulation, their commensal relationship with the cowbirds and their ichorous blood (although they're bipedal, laterally symmetrical, have facial features in the places where you'd expect them in terrestrial animals, etc.) -- far too many seem to be just slightly different versions of earth species, such as the elk-like creatures called, imaginatively, "yelk" (and once, presumably in typo veritas, "deer"). In some cases there's not even a name-change.
To take a different example: While the lifecycle of the creatures called hoxneys over the great year is fascinatingly worked out -- they spend the cold centuries as a crystal chrysalis before re-emerging as the world warms -- this doesn't alter the fact that, for most of the relevant time, they're horses.
Of course, there are plenty of famous fantasy worlds, some regarded as Towering Achievements, no less, that are largely populated with familiar plants, animals, customs, emotions and so forth, but one expects sciencefictional worlds to be a bit more alien. Aldiss, seemingly as if to distract attention from this concern, in the second and third volumes especially gives people and places extraordinarily long and silly names: Gravabagalinien, Queen MyrdemInggala, Princess TatromanAdala, JandolAnganol . . . Me, I remained less distracted than irritated by the stratagem: it's difficult to relate to a character when you have difficulty sounding their name in your reading mind.
Despite all of these things, and despite some patches of decidedly bad writing and embarrassingly declamatory dialogue, I did come away from Helliconia with a definite feeling of awe at Aldiss's achievement. I do genuinely feel as if I've been immersed in another world for quite some while, even if my favorite "character" is neither any of the humans nor the planet itself but the helico virus. There are ideas all over the place -- some of them just throwaways, others emerging as di ex machina -- which is one mark of good science fiction (although not always of good fiction).
In other words, Helliconia isn't just an enormous book but one with a hell of a lot in it. Hence the long and scholarly essays I mentioned at the outset.