This was an interesting short novel about, more or less, an alien life form that creates living personalities out of people's memories. (I.e., reanimates characters from a person's past). One thing this book did very well was make the reader wonder about what kind of consciousness or atomic makeup makes something a person. This alien re-creates the main character's long-dead wife. She seems normal in most other aspects, but has no memory of the last 10 years (including having killed herself), and doesn't understand how she got into her situation. I thought the book did a great job respectfully exploring the topic of what constitutes an identity. If it were possible to duplicate every cell in my body perfectly, would that person be me? If I could use my genetic material to create another person, would that person be me? If I had a stroke that caused me to lose my memory of the last 5 years, would I be the same person I had been before the accident?

Solaris creates a great narrative around these kinds of questions. The best passage in the book, in my opinion, is below.

"Listen", she said, "there's one other thing. Am I. . . really like. . . her?"
"You were", I said, "but now I don't know any more."
"What do you mean". . . ?
She got to her feet and looked at me with eyes wide open."You've already taken her place."
"And you're sure it's not her but me that you. . . Me?"
"Yes. You. I don't know. I'm afraid that if you were really her, I'd not be able to love you."
"Why not?"
"Because I did something terrible."
"To her?"
"Yes. When we were..."
"Don't say."
"Why not?"
"Because I want you to know that I'm not her."

The second thing Solaris does well is generally describing alien intelligence. Most sci-fi novels (I suspect, as I haven't actually read many) would apprach alien intelligence in a very anthropomorphic way. In Solaris, Lem makes it clear that we just can't understand alien intelligence, it is an altogether different thing -- it may not even appear to be 'intelligent'.