Penguin book covers are iconic. There is a nice article by Peter Campbell on the design of Penguin books in the London Review of Books. This is on the occasion of a forthcoming exhibition at the V&A [V&A press release in Word].
Market forces made the covers both livelier and more intrusive: they became advertisements. Penguin Classics (modern and ancient), where the Penguin name still means something, has kept a series style. Some of these covers are masterpieces of picture research. In 1984 the Pelican imprint was discontinued. The disappearance of the visual puns and graphic conceits which had given a modern gloss to worthy, often didactic non-fiction took Penguin one more step away from the BBC-like status it had once had. The illustrations in Baines's book prove how good, or at least how memorable, the designs were. Most middle-aged readers will recognise a remarkable number of them as books they knew, or books they have on their shelves, mellowing into crisp, brown-edged age. [LRB | Peter Campbell : At the V&A]