Which innovative book design feature, first widely popularized in the 1930s, was specifically created to allow readers to identify books more easily on store shelves?

The publishing industry has seen remarkable innovations in book design throughout history. From illuminated manuscripts to modern minimalist covers, product design in literature has evolved dramatically. Publishers invest heavily in cover design as it significantly impacts consumer purchasing decisions. This poll tests your knowledge of a groundbreaking product design innovation that revolutionized how readers interact with literary works.

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Dust jackets with plot summaries
Book spine titles printed horizontally (readable when shelved)
Color-coded genre indicators
Author photographs on the back cover