I read this book before the recent movie came out with George Clooney. The movie was decent, but poor Lem would be rolling in his grave if he knew Clooney had played a character he had penned.
Stanislaw Lem is all about the term "alien". If something is truly alien, then we as humans are never supposed to understand it. Once we understand it, said "alien" is no longer "alien", it has been defined in Human terms, and when something is defined in Human terms, it is no longer alien. Confused yet?
Therefore, most Hollywood and book aliens are nothing but Humans personified, and in his translated Polish he would say: "that's a load of crap."
Therefore, all his alien's are "alien." Humans do not understand them, and they never will. In "Solaris," the alien is a "planet" with an everchanging vast sea. On this planet, a team of researchers have established a base, and on this base, strange things start to happen. Long dead friends and relatives appear, as if nothing to has happened, though with no memory. It is reasoned that this is Solaris' means of communication, but whether malignant or benign, no one knows. Nonetheless, the Human reasearchers cannot take the strangeness and start to go strange themselves.
"Solaris" is a masterpiece of what the word "alien" really means. Lem had a belief, that aliens should be "alien", and he stuck to his Polish guns. If one is looking to learn how to create non-Star Trek aliens, There is no one better to learn from then Lem.
Having said all this, Lem is a dense read. If "Solaris" wasn't relatively short, it would be somewhat hard to get through. Lem likes thick sentences, and even in translation (I can't imagine what the text is like in Polish), the writing can be difficult. However, this should not deter writers and readers from experiencing a truly original mind at work.