Wow, this is something. Penguin has now, really finally, put together this box of 100 postcards, called Postcards from Penguin (£14.98), featuring Penguin covers from the past.
From a reliable source, you will find some of the very first Penguin covers with their broad colour bands and charming typography in the selection. There are also many postcards that feature illustrated book covers, like the lovely 60s paintings by Alan Aldridge on The Company She Keeps and There Must Be a Pony; the seismic David Pelham illustration for A Clockwork Orange; the perceptively light touch of Derek Birdsall on the covers for Chosen Words or Kiss Kiss; or the utterly Penguin crime covers with illustrations by Romek Marber on books such as The Case of the Dangerous Dowager or The Case of the Howling Dog.
Sometimes, you definitely should judge a book by its cover.
The box can be pre-ordered at amazon.co.uk. Of course, I've no intention to write on it, let alone mailing it out, or whatsoever.