Which innovative product design feature, first widely implemented in the 1930s, revolutionized paperback books and is credited with making literature more accessible to mass audiences?

The world of book publishing has seen remarkable innovations in product design over the centuries. From illuminated manuscripts to modern e-readers, the physical form of literature has evolved dramatically while maintaining its essential purpose. This trivia question explores a significant design innovation that revolutionized how readers interact with books. Test your knowledge about the intersection of product design and literature!

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Glued bindings that eliminated the need for stitching
Eye-catching illustrated cover art replacing plain text covers
Standardized page dimensions (the 'pocket size')
Acid-free paper that prevented yellowing over time