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It used to be so easy to define what a book was: a collection of printed pages bound inside a cover (hard or soft) that you could place on a shelf in your library, or in a store. Now, there are e-books, and blogs that turn into books, and long pieces of journalism that are somewhere between magazine articles and short books — like the recent opus written by author John Krakauer, published through a new service called Byliner — and a whole series of ongoing attempts to reimagine the entire industry of writing and selling books. If you’re an author, it’s a time of incredible chaos, but also incredible opportunity.

Byliner is one of the most recent entrants into the micro-publishing field, offering a selection of longer works by well-known, non-fiction authors such as Krakauer, who wrote a long magazine-style article about the alleged irregularities involving a charitable effort by fellow mountain climber Greg Mortenson. The piece was available as a free download for the first 72 hours — and saw more than 50,000 copies downloaded — and then was expected to become a paid download. Byliner said it’s planning to publish original works soon by authors William Vollmann and Anthony Swofford as well.

In effect, Byliner is a publisher just like Random House or Macmillan, but it is going to publish small runs of e-books, like a micro-imprint would at one of the larger publishing houses. Because it’s online only, however, Byliner’s costs are likely orders of magnitude lower, and it shares the revenue from the books 50/50 with the author. In a way, the service is positioned midway between the magazine industry and the book-publishing business. (Byliner CEO John Tayman is a former editor for Outside magazine.)

The site joins another boutique e-book publisher called The Atavist, which launched earlier this year and also focuses on long-form journalism — something between a magazine article and a short book-length project. But the Atavist has taken a real new-media approach, by offering its content through mobile applications for the iPhone and iPad (s aapl), as well as offering multimedia (all stories are available as audio versions as well as print) and Kindle and Nook versions.

These two new ventures join a market where Amazon (s amzn) is already publishing what it calls “Singles,” or short book-length publications that virtually anyone can produce. To take just one example, blogger and Hunch.com founder Chris Dixon recently bundled all his blog posts about venture capital (he’s also an active angel investor) and published them as an Amazon e-book. Journalism professor Jay Rosen mused on Twitter about doing the same thing with his blog posts about the future of media. And the list of publishers grows every day, with the TED conference launching its own e-book imprint recently, and marketing maven Seth Godin starting a new micro-publishing venture (backed by Amazon) called Domino.

Meanwhile, some authors are making millions by self-publishing multiple inexpensive e-books: Amanda Hocking became famous in the industry over the past six months for making over $2 million by self-publishing a dozen fiction books for younger readers, and recently signed a hefty contract with an existing publisher based on that success. Others have gone in the opposite direction; author Barry Eisler, after publishing a number of books through the traditional route, said recently he’s going to start self-publishing, because he will have more control over the process and will keep more of the revenue.

After centuries of not changing very much at all, the book industry is going through the same kind of upheaval as newspapers, Hollywood and the music business are — and that means more uncertainty, but also more opportunity.

Post and thumbnail photos courtesy of Flickr user Marcus Hansson