Rather creepy (as you'd expect from Lem) hard SF about a researcher who finds that his dream assignment isn't quite what he expected. I particularly liked the way Lem weaves the backstory into the plot. The translation is a bit clunky in places, and there's a lot left unexplained at the end of the book, but it's still an interesting read.
ISBN 0571219721. Search for this book on Wikipedia or isbn.nu.