With e-reader prices dropping like a stone and major tech players jumping into the book retail business, what room is left for publishers’ profits? The surprising answer: ads. They’re coming soon to a book near you.

To understand why this is inevitable, consider the past few years. The historically staid and technology-averse publishing ecosystem has been ripped apart and transformed.

Take the first seven months of 2010 alone: In April, Apple came out with the iPad and quickly sold over three million units. Apple also launched its own proprietary bookstore, iBooks. In June, Barnes & Noble lowered the price of …

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