Helliconia. Helliconia Spring, Helliconia Summer, Helliconia Winter

Helliconia is a planet that, due to the massively eccentric orbit of its own sun around another star, experiences seasons that lasts eons. Whole civilisations grow in the Spring, flourish in the Summer and then die in the brutal winters. The human-like inhabitants have been profoundly changed by their experience of this harsh cycle. In orbit above the planet a terran mission struggles to observe and understand the effects on society of such a massive climatic impact.
Massive, thoroughly researched, minutely organised, full of action, pulp references and deep drama this is a classic trilogy.