Which revolutionary pricing strategy, introduced in 1935, made quality literature more affordable to the masses with its distinctive colored covers?

In the publishing world, pricing strategies can make or break a book's success. From hardcover releases to paperback editions, from e-books to special collector's items, publishers employ various pricing techniques to maximize profits and reach different reader segments. This trivia question explores an innovative pricing strategy that revolutionized book accessibility and marketing in the early 20th century.

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Penguin's Paperback Revolution (priced at sixpence, equivalent to the cost of a pack of cigarettes)
Random House's Dollar Book Club (subscription-based model with monthly selections)
Harper's Sliding Scale System (prices decreased as sales volume increased)
Oxford's Academic Discount Program (tiered pricing based on educational institutions)