The e-book market may have just hit its tipping point: Barnes & Noble, the world’s largest bookseller, has acquired indie e-book seller Fictionwise for $15.7 million in cash. It’s part of the company’s plan to launch an e-Bookstore later this year. Fictionwise, which runs a trio of sites—Fictionwise, eReader and eBookwise—will operate as a standalone business unit under founders Scott and Steve Pendergrast.

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Why not Amazon? Scott Pendergrast told Teleread that he’d shopped the New Jersey-based company to a few buyers; one of the sticking points was that the new owner had to be willing to uphold Fictionwise’s DRM-free, multi-format e-book software standards. (Amazon’s insistence on using its own, proprietary file format instead of an open standard like Epub, has garnered criticism from tech expert Tim O’Reilly, among others, per Forbes). Fictionwise offers fiction and non-fiction, including best-sellers from authors like Dean Koontz, Tom Clancy and Janet Evanovich; publishers can choose to offer e-books with or without DRM protection.

More after the jump.