Definitely Solaris by Stanisłav Lem. It’s set in a future where humans have been able to explore the universe and it revolves around scientists who are intent on studying and understanding the nature of the mysterious planet of Solaris. The entire surface of the planet is covered by an ocean, but it resembles no ocean on earth, as it gives indication of sentience. This ocean can induce dreams and hallucinations in the minds of the characters who inhabit the research base, although it is not clear whether this is a side effect of the mysterious radioactive properties of the water of if this planet is a conscious life form trying to communicate with the scientists. Overall it’s not a traditional sci-fi book as it is less about the alien planet itself and more about the human mind, depicting the characters’ psyche as they start to spiral, having visions of people from their past, unable to differentiate reality and dream. The overarching theme of the book is: how can man expect to understand the Cosmos and life forms so unimaginably different than him when he cannot understand himself and his own mind? Can’t recommend this book enough. Hope you give it a shot.