We keep seeing this over and over again in the music and publishing world (and it's mirrored in examples from other industries): when you let people taste, sample, borrow, purchase through loss leader pricing, even "shoplift" product, if they like what they see they will come back and buy more.
In this case, people who read books through the Kindle Lending Library buy more books. Even books not in the Lending Library see a sales increase if they are part of a series that is. #ddtb
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Kindle Owners’ Lending Library Drive E-Book Sales Increase | Digital Book World
Sales for books available to Amazon Prime subscribers through the Kindle Owners’ Lending Library have increased over the two months of the program’s existence.