I've bought a new mug!
When Penguin first published their books in 1935, they had very plain covers for the instantly popular and cheaply priced paperbacks. For many years (off and on) I have been collecting "Pink Penguins", officially Cerise, which denoted Travel & Adventure. The main other colours were Green for Mystery & Crime and Orange for Fiction, though Penguin No.1 was Dark Blue for Biography: Ariel (A life of Shelley) by Andre Maurois.
By the late 1950's plain covers gave way to pictures but still with a penguin in a cerise background on the spine, then by the mid-1960's the familiar penguin logo was all that remained; colour coded no longer. My collection started many years ago with the two volume edition of Apsley Cherry-Garrard's The Worst Journey in the World which led me to an enduring interest in polar exploration. I now have a nearly complete set of the Travel & Aventure Penguins and many other Polar books in my collection.
Together is interesting. Published first in hardback in 1923 (Penguin Edition No 502 in 1945) Norman Douglas has a companion "Mr R." which, unknown to the general reader, was actually a 15-year-old Italian, René - Douglas was 55 by then, though the book is not in any way overtly homosexual; that would have been impossible to publish in Britain at that time. It is an account of their travels in Austria. In another of his books Fountains in the Sand he travels to Tunisia which he portrayed as formerly prosperous in the days of the ancient empires, but at the time of his visit in 1912 sadly decayed, due in Douglas's view to the "fatalism of Mahomet and his religion".