Stanislaw Lem, The Invincible

A spacecraft lands on a distant planet, in search of answers to the mysterious disappearance of an earlier spacecraft and its crew.  At first the planet seems relatively benign.  They soon notice a few puzzling things, but nothing that seems likely to be really threatening.  But the puzzling things become more puzzling, and they start to realise they’re in the presence of something very alien indeed.

Stanislaw Lem’s The Invincible, published in 1964, explores similar themes to Lem’s better-known novel Solaris. Lem had a gift for creating aliens that are truly alien, so alien that we’re not even sure that they’re actually alive in the sense that we understand the word. They are so alien that any kind of communication with them is virtually impossible. They appear to be intelligent, but it is intelligence of a kind that we can never hope to understand. Their intelligence is not necessarily superior to ours, it's simply different in the most profound way.

It’s a gripping read, very disturbing. This is superb ideas-based science fiction.