What do you understand by Shiling serious?

Nineteenth-century periodicals serialised important novels, which gave birth to a particular way of writing novels. In the 1920s in England, popular works were sold in cheap series, called the Shilling Series. The dust cover or the book jacket is also a twentieth-century innovation. With the onset of the Great Depression in the 1930s, publishers feared a decline in book purchases. To sustain buying, they brought out cheap paperback editions.