The penguin logo came into being in response to the publisher's desire for a design that was "dignified yet flippant." That perfectly sums up the attitude Penguin Books has kept up to this day.
Evolution of the logos for the various Penguin Group Imprints (e.g. Puffin, Pelican, Peacock)
(from Phil Baines' magnificent book Penguin by Design)
Penguin's Production Manage Hans Smoller's study of the origins and application of Penguin and Puffin devices.
(David George Pearson on flickr)