Which packaging innovation in the publishing industry, introduced in the 1930s, revolutionized book marketing and accessibility, leading to a boom in mass-market reading?

Book covers have evolved dramatically over centuries, from simple protective bindings to sophisticated marketing tools. Today's book packaging incorporates innovative designs, materials, and technologies that not only protect the content but also serve as powerful marketing elements. This trivia question explores a significant packaging innovation that revolutionized the publishing industry and changed how readers interact with books.

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Paperback books with illustrated covers
Leather-bound embossed covers with gold leaf
Transparent dust jackets with author photographs
Hardcover books with removable bookmarks