I do like old science fiction book covers- not just Penguin, but all the more lurid publishers as well. And then there's the feverish world of the pulp magazines etc. For me the classic era is the 1960s and 70s- the move away from quasi-imperialistic and rather conservative fantasies involving spaceships and aliens towards more warped, anarchic visions of disturbing dystopias and chaotic chronicles of inner space. But still - some great art produced in the 80s - with it's characteristically airbrushed look- and more recently as well.
That Harry Harrison book is a superb novel of interspecies conflict in a time of climate change. The 'toothy dolphin' on the front is in fact a reptile rather than a mammal-it's, ahem, a genetically modified plesiosaur used as a marine transport vehicle by highly evolved and human-hating saurians. I'd definitely recommend getting the first book in the trilogy-this is my copy in the same series:
They're illustrated throughout with superb woodcuts, which bring this alternative Pleistocene world to vivid life. The best stuff on the list is fairly far from the magazine SF novels which would have been the likely contenders for a Hugo equivalent at the time -- it'd probably have been a competition between E. E. 'Doc' Smith's Galactic Patrol and Jack Williamson's The Legion of Time. 1938 is just before John W Campbell made Astounding Science Fiction a less pulpy magazine, and some years before the average quality of prose in magazine SF really rises.
If you're into Old/Vintage SF, read on.