Gordon Burgett is a publishing industry sage. Here's a link to his discussion of a report on the future of digital publishing.
An important point--the report notes that any digital product will be able to stay current, because it can be updated quickly and easily. But, a print product will become dated.
This is very important. If you are writing a book with a lot of links, or even with an addendum with references, these can quickly become obsolete. Even product references can become antiquated. I've experience this as people who own copies of Scrapbook Storytelling have emailed me trying to find various suppliers who, alas, are no longer in business.
Now that doesn't stop me from adding product references in my books, but it does mean that the public's ability to contact you or to visit you on your website takes on new importance. People do know how to contact you, right? Make it as easy as possible, is my mantra. Even when I email people back to to say, "Sorry, those suppliers are no longer in business," I've taken the time to further my relationship with the person writing the email. Usually people will tell me how surprised they are that I personally have responded.
Why shouldn't I? Oh, sure, it takes time, but I figure it's the least I can do. They spent money and time with my product. I owe them a response.
Also note that POD (print on demand) technology continues to improve. Many years ago, a Japanese concept call JIT (Just In Time) sourcing revolutionized the auto industry. With JIT, a car manufacturer such as Toyota could order parts and know they'd arrive "just in time." This saved the cost of warehousing. Now, you might think, "So what?" But let's use a little imagination, shall we? Suppose are an American manufacturer who has ordered all the parts to build a month's worth of cars. You have to unload those parts from a carrier (+ labor/workman's comp for a job prone to injuries), warehouse them (+ labor, + insurance on the building, + inventorying supplies, + knowing where the supplies are, + rent), store them until used (+ cost of overhead and any damages if there's a leak in your roof, insurance), then ship them to your factory floor. Compare this to: the parts arrive and are taken directly to the factory. Hello!
It's the same with books. If you print them all in advance, they must be boxed and shipped to your warehouse. If the warehouse is damp, if the books get rained on during shipment, if the number of books in the book isn't the correct amount to send out in any one order (and they need to be repacked), you have incurred costs in addition to publishing your books.
Oh, don't look for everything to change overnight. Some time ago, the music industry introduced kiosks where folks could order sheet music and have it printed as they waited. The prediction was that there'd be a kiosk in every music instrument store. It hasn't quite happened. But on the other hand, more and more scrapbookers are storing their photos digitally and printing them out as they need them, either at home or through vendors.
There's a change coming to this industry. It bodes well for all of us...but as to who will be the big winners and losers, well, we'll just have to wait and see.