Perhaps the biggest stumbling block to wide-spread digital publishing is digital rights management. The current major players (Apple, Microsoft, et al) have one model of how that should work. Their model has significant problems (IMHO). But it's not the only possible model. Read on for an alternative.
Traditional ownerships of things like books and music involve a one-time transaction. Once you've bought it, your relationship with the author, publisher, and bookstore ends. There's no on-going contractual relationship. There is copyright law that limits what you can do with the item you've purchased, but that is a society-wide law, not a contact between you and the publisher.
Current digital rights management (DRM) schemes from companies like Apple and Microsoft work differently. They imply a permanent relationship with the publisher. Without it, you can't move the material to a new computer, or to a new version of the player application. Because usability depends on this long term relationship, if the company ever disappears or drops support for the technology, your ownership suddenly evaporates. This issue is at the heart of why I don't buy DRM'd works. The future of my ownership is simply too foggy.
But digital rights are important. So long as our society uses capitalism to allocate resources and motivate its members, ownership is virtually inescapable. Ownership of digital works, without some sort of DRM to back it up, is tenuous at best. So what to do?
I believe the answer is to change the model. Don't sell digital works, lend them. Rather than the model of a bookstore, let's try the model of a private lending library. Rather than thinking Amazon, think Netflix.
Using the library model entirely changes your expectations of digital rights management. For example: It's now OK if the work is only usable on the device it was downloaded to, because you can always check it out a second time on another device. It's now OK if the work is only usable for a limited period of time, because that's how borrowing from a library works, and you can always renew the check-out. It's not great if the library goes out of business, but if it does, you don't lose something you'd paid for and thought you owned permanently.
And it enables other aspects digital works that were not a feature of the traditional publishing world. In the old days, once something was printed, it was finished. But today, works can continue to grow, interlink, and be annotated indefinitely. For example, I find the MySQL manual somewhat opaque, but the online version has annotations from users that almost always answer the question I had clearly and with explicit examples. With the lending library approach to DRM, if I check out the same title at different times, it's just natural to get the most up-to-date version each time, not one that was frozen at some arbitrary point in time by the publisher.
A lending library works best for books, but it can also work for music, video, and other forms. The higher the bandwidth between the library and the user, the better it works for these other media, since they tend to require more frequent borrowings of larger sizes. Luckily, bandwidth is constantly growing.
So now that I've figured out "the answer," how do we proceed? Implementation is not hard. What's hard is convincing someone in a position to bring on-board the existing publishers. The point to this essay is to get the idea out where someone in that category may find it. Please let me know if you do. I'd love to be in the beta, or even help develop the system.