Apparently, there were two major creative forces that drove Penguin's innovation of book design: Jan Tschichold in the 40s and Germano Facetti in the 60s and early 70s (with Romek Marber playing a key role).
To the left is one from the early 40s, under the tutelage of Tschichold. I like it. It's kind of cute. To the right is a Penguin Crime cover done by Marber, as commissioned by Facetti. This I REALLY like. And by the way, I really love the Maigret detective series done by Georges Simenon (Maigret and the Headless Corpse is one of my all-time favorite books).