On occasion, old science fiction can easily take the Terrible Book Cover cake.
For example, I present the excellent book John Grimes: Tramp Captain by A. Bertram Chandler.
What an intelligent, well thought-out title for a great work of futuristic fiction. I'm sure the illustrator was only helping to promote the book's already stand-out intellectual ideas. Why, I'm surprised this one didn't win the National Book Award.
HA! Oh, man. I just couldn't get through that whole thing with a straight face. He's the captain of what now? Alright, let me try again.
Phule's Paradise by Robert Asprin clearly seeks to change the way sandy-haired surfer dudes and scantily clad young women are portrayed.
After all, everyone knows that when faced with a giant green boar-man, the surfer dude will not just turn around and hit the waves. No, he will stand up for what is right by punching the monstrous creature in the tusks with a ridiculous expression on his face. And then the girl who was waitressing in a bathing suit will smack him over the head with her serving tray, because she knows surfer dudes who punch boar-men are up to no good.
At least the tagline acknowledges the absurdity here: "In space, no one can hear you laugh."
Which brings us to Harry Harrison's The Stainless Steel Rat Goes to Hell.
Here, you note immediately that women are being respected in this book. We also know why the Stainless Steel Rat is called such - a) his costume, and b) he's in hell. We all know rats are not quite God's chosen critters.
This book must also feature a great deal of reptilian life - not only do we have a dragon on the cover, but also two snakes. So comfortable is the Stainless Steel Rat with all those teeth and hissing that he's having a Bond-style martini, while he ignores the pitchfork at his throat. Of course, that thing probably hasn't been sharpened in millennia; devils are known slackers.
But then what's with the guy whose hands are wide open by the Rat's head? Is he playing peek-a-boo?
By the way, all of these books are series. I think it's because the illustrators wanted to do more covers, not that the authors wanted to write more books.