Which retail product development strategy did Amazon first test with its Kindle e-reader that revolutionized digital book sales?

The business side of literature has evolved dramatically over centuries. From traditional publishing houses to online marketplaces, the way books reach readers reflects changing retail strategies and consumer behaviors. This poll tests your knowledge about a fascinating product development strategy that revolutionized how we consume literature. Are you familiar with the business innovations behind your bookshelf?

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The subscription model allowing unlimited access to books (Kindle Unlimited)
The 'Whispersync' technology that lets readers switch between reading and listening
The 'loss leader' pricing strategy selling e-readers at cost to profit from content
The direct publishing platform allowing authors to bypass traditional publishers