Symbolism, Imagery, Allegory
Kelvin reads a lot in Solaris. And let's face it: A lot of the books he reads are pretty boring. On and on he goes, telling us about how this Solarist thought this and that Solarist thought that, but then they decided they were wrong and we don't actually know anything about Solaris. And then he picks up another book and it says that this Solarist thought this and that Solarist thought that and… well, you get the picture.
So what gives? The reason we need these boring books is that Lem thinks they're funny. And the reason he thinks they're funny is that the world is filled with boring books. Boring books of science, in which scientists say how they think this and that this other scientist thinks that, and then they decide they're both wrong and don't know what to think. All the science books are, if you will, fiction. They're science fiction that insists on not being seen as science fiction. Which is actually pretty funny if you think about.
So the Solarist books, and Kelvin's careful, exasperating attention to them, are a joke. They're a joke on science, which often doesn't know what it's talking about, and they're a joke on science fiction, which never knows what it's talking about, even though it pretends it does. Hardy har har—everyone takes themselves so seriously, right? And in this sense, the Solarist books are also Lem making fun of himself, of his own effort as well as his own product.