One could say (and I would) that the e-reading revolution has had its fourth anniversary, since it was late November 2007 when Amazon first released the Kindle.

There had been dedicated e-reading devices before then, including the Sony Reader—in the market when Kindle arrived and still here, if not wildly successful—and the already defunct Rocket Book and Softbook devices that had debuted and disappeared some years before. And in the early 1990s we had the Sony Bookman, which showed only a few lines of text at one time and disappeared with barely a trace. Before Kindle, the biggest-selling e-book format put content on the Palm Pilot, and the total e-book market was so far beneath a rounding error that any investment by a publisher in digitization was being made on faith, not on commercial evidence.

And many people in publishing believed that reading on a screen would take many years to take hold, if it ever would.

Now, four years later, we are living in a changed world, although not yet a transformed one. But transformed might be coming very soon.

E-book sales in the United States now appear to have reached the 20-percent-of-revenue threshhold at some publishers already (so it is there or will be for everybody very soon), so there are some things we can say we know about the shape of the future, but some very important other things that we don’t know yet.

• We know that most people will adjust pretty readily to reading straight-text-narrative books on a screen rather than paper.

• We know that parents will hand their iPad, iPhone, or Nook Color device to a kid so that the kid can enjoy children’s books on the device.

• We don’t know whether adult illustrated-book content will be equally well accepted by book consumers on devices, even though there are more and more devices capable of displaying pretty much what publishers deliver on a printed page.

• We don’t know what parents will pay for a brief illustrated children’s book delivered for a device, but it appears it might be much less than they’re willing to pay for paper.

• We know that consumers will pay paperback prices and more for plain vanilla, aka “verbatim,” e-books.

• We don’t know whether consumers will accept paying higher prices for video, audio, or software enhancements to the verbatim e-books.

• In fact, we don’t know if consumers would pay paperback prices for e-books if the paperback were not ubiquitously on sale as a benchmark for pricing.

• We know that e-book uptake, as measured in sales or the percentage of publishers’ revenues, has doubled, or more than doubled, every year since 2007.

• We know that it is mathematically impossible for that rate of growth to continue for even three more years (because it would put e-books at 160 percent of publishers’ revenues if it did!).

• We know from announcements about new devices and a recent Harris poll predicting increased device purchasing that there are no expectations for a slowdown in e-book adoption anytime soon.

• We don’t know if we’re going to find a barrier of resistance, or perhaps we should call it the barrier of “paper-insistence,” at some sales level over the next two years (at the end of which e-books would be 80 percent of publishers’ revenues at the growth rates we’ve seen over the past four years).

• We know there’s a big and developing market for English-language e-books globally, as the e-book infrastructure builds out in markets around the world.

• We don’t know how quickly those markets will develop or how big they can ultimately become.

• We know that the number of bookstores suffered a sharp reduction in 2011 because of the Borders bankruptcy.

• We don’t know if the remaining bricks-and-mortal retail network—the bookstores led by B&N and including the independents as well as the shelf space devoted to books by the mass merchants—will get a second wind from the disappearance of the Borders competition, buying publishers some temporary stability in their store network, or if the erosion of shelf space will continue (or even accelerate).

• We don’t know what the loss of bricks-and-mortar store merchandising will mean to the ability of publishers and authors to introduce new talent to readers, or even just to introduce a new work by established talent.

• We don’t know if improved book discovery and merchandising are compatible with the application of “scale” by publishers outside of vertical niches, be they topics or genres.

• We know that agents and authors will accept an e-book royalty of 25 percent of net receipts in today’s environment, where 70 percent or more of the sales are still made in print.

• We don’t know if the threat of the alternative publishing options will force that royalty rate up if sales fall below 50 percent print or 30 percent print.

• We don’t know if sales falling below 50 percent print or 30 percent print is several years away or much less.

• We know that the EPUB 3 standard and HTML5 enable app-like features to be delivered as e-books.

• We don’t know if those features will make any commercial difference for the straight text content, which is the only commercially proven e-book type.

• We know that companies that are not book publishers are using the relative ease of publication of e-books to deliver their own branded content to the e-book marketplace.

• We don’t know if book publishers will develop an e-book publishing expertise that will persuade those brands to go through them, the way the brands have in the print book world, rather than disintermediating them.

Since I have been expressing my concerns about the impact of the e-book revolution on general trade publishing, which I have been doing with dramatic intent since six months before the Kindle at the BEA in 2007, I have been saying the general trade houses have to get audience-centric (which means choosing content to fit vertical niches).

Now I am adding another urgent suggestion to general trade publishers: reconsider your commitments to publish illustrated books in any time frame more extended than a year or two and think about sticking to straight text, unless you have paths to the customers for those books that do not go through bookstores.

If we do end up in an 80 percent e-book world anytime soon—and we very well might—you’ll want to own the content you know works (for the consumer) in that format, not what you don’t know works any way other than in print.

For children’s books, the key is brand. There will be demand for Eloise and Madeline and Alice in Wonderland for years to come, but the product and pricing equations could be totally up for grabs.

Mike Shatzkin, founder and CEO of The Idea Logical Company, has been an industry consultant for three decades. His blog, The Shatzkin Files (idealog.com/blog), is the source of this article, and a compendium of the first two years of blog posts is now The Shatzkin Files, Volume 1, available as an e-book and in a print edition. To reach him, email mike@idealog.com.