Penguin Handbooks, of which the above is the 1948 revised reprint of a 1944 title, began during WWII as part of the effort during the war to get the British public to produce more food for self-consumption.
Titles under the Pelican imprint were viewed as educational titles. The first, shown here, was a 2-part publication of Shaw's 1928 text, with additional chapters. The Pelican versions retains the dedication to his sister-in-law, "The intelligent woman to whose question this book is the best answer I can make". The imprint was discontinued in 1990.
The 1960 publication of "Lady Chatterley's Lover" shows the change to vertical bands, and the introduction of illustrations and photographs to the covers. Penguin was famously charged with obscenity for publishing this title, a case which Penguin won and which marked a turning point in the censorship laws in Great Britain.
Penguins published in New York differed in design, font and size from those published in the UK. The above title, Penguin 511 by Eric Ambler, is a 1945 edition of a 1942 title. Although retaining the penguin icon, the American covers used illustration from the beginning, and were shorter than their UK counterparts. American Penguins make up the bulk of the Fales Penguin paperback numbers, although not all are signed by the author, as this one is.
The Penguin icon, chosen for being 'dignified but flippant', according to the Penguin Group website, has changed over the years: sometimes it stands alone, sometimes there are dual penguins, but my favorite is the dancing penguin, shown above.