The book is superb.
But to appreciate the book have to understand that Lem, in his serious mode (he also wrote comedies and light-hearted adventures, which are quite different), is not very concerned about plot, nor about most things American sci-fi has always been about: There are no heroics, no politics, no space wars, no McGuffins, and there's only minimal world-building.
Lem is an engaging writer, but also a somewhat dry one, and Solaris is a sort of essay on science, camouflaged as a novel: Like his other masterpiece, His Master's Voice, it is a critique of science. Whereas HMV attacks the scientific field pretty broadly, Solaris is more of an investigation into the limitations of the human mind: A scientist visits a far-away research station and comes to realize that all previous research (by decades of so-called "Solarisists") isn't even wrong, it's irrelevant. About a third of the book is spent on a wry summary of all the scholarly research that Solarisists have been performing and collecting in journals. Even after many decades, Solaris has funded many scientists, and yielded endless observations, but also zero insight. (At the time the book is set, most scientists have simply given up, and the station is largely abandoned.) Solaris is an inscrutable alien organism that the human mind can simply not understand. Ironically, even the main character comes to think, towards the end, that he has understood Solaris, when it's obvious to the reader that his conclusions are just more of the same, a reflection of our human-ness.
Of course, both movie adaptations got this wrong, or chose to ignore that aspect of the book. They think it's about people reincarnated from memories, and that it's about love. In the book, the "ghosts" are actually dealt with rather simply and matter-of-factly; the scientists find themselves being tormented by them, and quickly move to destroy them. Lem, however, doesn't bother with the emotional melodrama you find in both movies.
I don't remember what Soderbergh's version aimed for exactly (other than being a sort of Last Tango in Paris in space), but Tarkovsky's is very much inward-looking and concerned with the nature of artifice and truth (the "ghosts" are not real, and neither is Solaris, in a way). Tarkovsky didn't really want to create a sci-fi movie, and so you get something which is not entirely convincing.