The Baltimore Sun commemorates an important birthday
"The first Penguin paperbacks appeared in the summer of 1935 and included works by Ernest Hemingway, André Maurois and Agatha Christie. They were colour coded (orange for fiction, blue for biography, green for crime) and cost just sixpence, the same price as a packet of cigarettes."
More links on the The Paperback Revolution
IOBA presents a far more detailed account of the history of the paperback.
"In fact, the first bona fide mass-market paperback in the English speaking world is said to be Malaeska , by Mrs Ann S Stephens, which was published in June 1860 by the pioneers of the Dime Novel, Erastus and Irwin Beadle."