The cover art for all of the following covers is uncredited, but some are signed by the artist, Stanislaw Fernandes, and near as I can tell, he produced the cover art for the other books in the series, too:
[CLICK IMAGES TO ENLARGE]
P.S. The above books are all from my personal library and were scanned by me just a few minutes ago. Of course.
P.P.S. I often buy vintage books solely for the cover art. I bought Stanislaw Lem’s books (years ago) because I wanted to read them. And I kept them because I wanted to re-read them. Because Lem is now one of my favourite writers.
Keywords:The Investigation, The Futurological Congress, The Star Diaries, The Cyberiad: Fables for the Cybernetic Age, Tales of Pirx the Pilot, Mortal Engines: Electronic Escapades in the Lands of Tomorrow, Return from the Stars.