Which groundbreaking design element did Penguin Books introduce in 1935 that revolutionized the publishing industry and remains iconic to this day?

In the publishing world, few brands have had as much impact as Penguin Books. Founded in 1935, this publisher revolutionized how books were marketed, sold, and consumed by the public. Their innovative approach to book design and distribution transformed literature from a luxury item to an accessible commodity for the masses. Test your knowledge of this literary institution that bridged the gap between commercial publishing and artistic integrity.

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Color-coded book covers (orange for fiction, blue for biography, green for crime, etc.)
First publisher to use recycled paper for all publications
Introduction of the first e-reader prototype called 'Penguin Pocket'
The 'subscription box' model where readers received monthly curated selections