Which packaging innovation in the publishing industry was specifically created during WWII to make books more portable for soldiers, later revolutionizing mass-market reading?

The publishing industry has undergone numerous transformations in how literature is presented to readers. Packaging innovations have played a crucial role in making books more accessible, portable, and appealing to different market segments. This trivia question explores a significant packaging innovation that revolutionized how people consumed literature in the 20th century and continues to influence reading habits today.

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Paperback pocket books with glued bindings
Leather-bound travel editions with thin pages
Spiral-bound waterproof editions
Folding accordion-style miniature books