Which innovative product development in the publishing industry is credited with making books widely accessible to the middle class for the first time in the 15th century?

The publishing industry has evolved dramatically over centuries, with various innovations revolutionizing how literature reaches consumers. From ancient scrolls to modern e-readers, the 'product delivery' of literature has shaped reading cultures worldwide. Test your knowledge about a pivotal moment in literary product development that transformed access to books and reading habits across society.

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Gutenberg's movable type printing press
The introduction of paperback books by Penguin Publishing
The creation of subscription-based lending libraries
The development of wood pulp paper manufacturing