Which revolutionary book packaging innovation was introduced by Penguin Books in 1935, making literature more accessible to the general public?

In the publishing industry, product packaging plays a crucial role in attracting readers and protecting books. Throughout literary history, certain packaging innovations have revolutionized how we consume literature. This trivia question explores a significant packaging development that changed the accessibility and popularity of books for the average reader. Test your knowledge of how literary works have been physically presented to the public!

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Mass-market paperbacks with standardized size and affordable pricing
Leather-bound collector's editions with gold-leaf lettering
First audiobook recordings on vinyl records
Subscription-based book delivery in custom wooden boxes