A few years ago, very few people had heard of ebooks, even though they have been around for decades. Several things kicked off the change that started book publishing on the path to digital books becoming a mainstream phenomenon. First was the invention of e-ink, as seen first on a Sony Reader, then the Kindle, then the (non-color) Nook ereader. The e-ink screen offered vastly reduced eyestrain, much better battery life, and no glare in bright light.
Second, Amazon got into the business with their Kindle ereader. The Kindle had two additional things going for it, in addition to e-ink. The first was wireless delivery of ebooks. You didn't even need to own a computer to buy and download books; you just needed a Kindle. The second was that Amazon had a much bigger, better organized ebookstore than Sony or Fictionwise, or any previous ebook vendor.
The combination of an easy to use ereader and a well-stocked store made ebooks take off. Barnes and Noble got into the act with the Nook and the Nook Color, and Apple came out with the iPad.
The Kindle and other dedicated ereaders hooked the voracious readers— people who read two or more books a month. The iPad hooked the casual reader. They already had the device, and it allowed them to download several different ereader apps, including Apple's own iBooks, and read books in color. Over the last year, ebook sales have been growing by leaps and bounds.
Which is the main reason that publishing solely in digital form is now a viable option! And that's why my first book, The Sixth Discipline is now in the Kindle, Nook, and Smashwords ebookstores.