Which e-commerce innovation caused the most significant disruption to traditional book publishing revenue models in the early 2010s?

The book publishing industry has undergone dramatic transformation with the rise of e-commerce platforms. What began with online bookselling has evolved into self-publishing platforms, e-books, audiobook subscriptions, and direct-to-reader marketing channels. Test your knowledge about how e-commerce has reshaped how literature reaches readers in the digital age.

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Amazon's introduction of Kindle Direct Publishing allowing authors to self-publish with 70% royalties
Barnes & Noble's Nook e-reader and associated online bookstore
Google Books' scanning project and searchable book database
Apple's iBooks store launch on the iPad