Things magazine have put together an awesome gallery of penguin book covers. Includes beautiful covers overseen by Jan Tschichold as well as the late typographer Hans Schmoller. My stand-out years are between 1961-1972 when Italian art director Germano Facetti was in charge of design. Facetti enlisted Polish graphic designer Romek Marber to redesign the look of the Penguin series and the rest is history.
Source: Ace jet 170