Solaris
by Stanislaw Lem (1961)

The central character in Solaris is a sentient ocean that humans find impossible to communicate with. The Earth space-station orbiting the planet is practically a death sentence, as everyone who serves on it seems to get the urge to commit suicide. Lem asserts we should accept that there are things we will never understand. Mind-boggling.

Dune
by Frank Herbert (1965)

The first and by far the best of Herbert's Dune series - to survive on sandworld Arrakis water resources must be carefully preserved. A political power struggle over an immortality drug sees the hero lead desert dwellers and sandworms into battle and begin his rise to messiah status. A sprawling saga that remains an immensely popular chart-topper.

Emphyrio
by Jack Vance (1969)

Much-beloved SFWA Grand Master who is best known for his 'Dying Earth' fantasy stories, often errantly categorised as sci-fi. The stand-alone Emphyrio edges out the popular Demon Princes series as his best sci-fi for its scathing attack on economic imperialism. In the far-future craftsmen on the planet Halma are prohibited from utilising modern technology… and the revolution begins.

Inverted World
by Christopher Priest (1969)

UK writer Priest's best genre-SF book is a unique perception versus reality tale about a man who leaves Earth to work as a Future Surveyor on an alien world. The hyperbolic planet proves to be 'inverted' - a place where normal rules of time and space have little relevance. Priest is now a noted writer of innovative non-genre fiction.

Downward To the Earth
by Robert Silverberg (1970)

A former colonial administrator harbours a guilt-complex over his time on the planet Belzagor. He returns to try and set things right and participate in the bizarre nildor  religious rite which he had suppressed in colonial days. An effective examination of colonialism that still bears significant relevance to the modern world. One of Silverberg's best.

The Fifth Head of Cerberus
by Gene Wolfe (1972)

Three connected tales - best read as a novel - focusing on GW's favourite subjects of identity and self-exploration. Set in a distant French-colonial bi-planetary system, the key players are a clone who confesses the story of his childhood and a shape-shifting alien who impersonates an anthropologist with ironically fatal results.

The Heritage Of Hastur
by Marion Zimmer Bradley (1975)

Transitional novel in the popular Darkover series, with Bradley moving away from writing for younger readers and tackling decidedly more serious themes. The relationship with the Terran Empire is at the heart of a struggle on Darkover, where the telepathic inhabitants  take sides in a conflict that pits science against religion.

The Dosadi Experiment
by Frank Herbert (1977)

Often regarded as Herbert's second-best novel (behind Dune), Dosadi is a toxic planet and the setting of an experiment designed to test the survival skills of any being unfortunate enough to be dumped there. A government agent of the ConSentiency - a federation of races dedicated to peace - is sent to uncover its horrors. More than worthwhile for anyone with an interest in Herbert's work.