Here's a 1954 Penguin paperback of Angela Thirkell's The Brandons, which originally sold in England for "two and six" (two shillings and six pence). Thirkell's novels, set in rural English villages with names like Winter Overcotes, are a kind of cross between Jane Austen and P.G. Wodehouse. I found this book at Monkey See, Monkey Read.
You can read about the history of Penguin paperback book covers at the Design Museum website. In 2006, the Design Museum ran an exhibition on "Designing Modern Britain," focusing on landmarks of modern British design—including the covers of Penguin books. The website says: "The rigorous application of colour, grid and typography in those early paperbacks instilled Penguin with a commitment to design from the start." There's also a book about Penguin covers: Penguin by Design: A Cover Story, 1935-2005, by graphic designer Phil Baines.