Len actually wrote a whole bunch books about humans meeting inscrutable aliens — Fiasco, The Invincible, Eden, His Master's Voice — but I think the best one is still Solaris. Many people know the film adaptations, but the novel is ultimately somewhat different in motivation and conclusion. It's a great read.

(One thing I just realized: For some reason Lem loves starting the book with a landing. All of these except His Master's Voice start with the main character(s) either landing a spaceship or docking with one.)