From the library of yours truly:
[CLICK IMAGES TO ENLARGE]
The foreground figures in the above (uncredited and unsigned) illustration are tremendously effective; the background figures, not so much.
Look Here: THE COSMIC ENGINEERS, with cover art by the great unknown
Ragged Claws Network 
Though not by Richard Powers, obviously someone influenced by him. I like it.
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This cover has been credited to Powers by a number of authorities, but it might actually be by John Schoenherr (my dad). His records show that in early 1969, he was hired by Paperback Library to do a cover for “Cosmic Engineers” – but I never got a chance to ask him if PBL used his painting or had someone else redo it. Even so, although Powers-like, this looks more like a Schoenherr to me: the muted palette, the texture of the background (similar to other works painted on gessoed illustration board or masonite at this time), the composition, the drawing of the foreground figures, the “heaviness” of the forms. On the other hand, the Powers covers I’ve seen from the late 60s and early 70s are generally much lighter, brighter, and more colorful. But I could be mistaken…
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Your argument for the attribution of the Cosmic Engineers cover art to your dad looks solid to me, Ian. Since I have never believed that the cover art was by Powers, I am inclined to agree with your stylistic critique, but the real clincher is your dad’s record of the commission from Paperback Library! Without that, your rejection of the Powers attribution would remain persuasive, but your positive attribution of the artwork to Schoenherr would be far less so.
Someone should tell ISFDB, as their catalogue record for the book currently affirms the attribution to Powers and includes the note “The artist is not credited, no visible signature [style is unmistakable].”
Thanks for posting!
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