As a Kindle owner for two months now, I've been exploring the availability of content. I've been impressed and, frankly, surprised at the sheer volume (sorry, no pun intended) of material available. I estimate that most of my library (covers many topics and types of books) is available digitally. What's more I think the Kindle can store them all.
I'm not exactly sure since acquiring many of these books electronically will cost money. But many are also free. Project Gutenberg is an excellent source. And many book sites, such as Amazon, Kobo, and Wattpad, offer free ebooks as well.
Now here's the catch. The quality of the e-books is not always the same. Example: I downloaded Alice in Wonderland from Project Gutenberg. To my surprise the ebook had no pictures with it. The picture are available elsewhere on the site but not integrated.
Then I checked my devices. I have a Kindle version and an epub version (an iBook from Apple). The epub version has pictures too but not the same ones from Project Gutenberg or in my paper copy.
Sigh. So if you think you can replace your library one for one with a digital version, you may be disappointed.
Additionally, some of my favorite books are only available in paper. For example, I have all 11 volumes of the History of Civilization by Will Durant. As far as I can tell, it only exists in hardcover.
Likewise, the Time-Life series of the Enchanted World is only available in paper. I suppose it is just as well because the Kindle only displays in black and white so the Enchanted World series would be hard to show well.
The other thing that I'm sure they never replace in books is the feel of them. I have many old books from the 1920's. Many in poor condition but that does not bother me. It is the feel of the old book that I like and in some cases what they old tomes show. For example, I marvel at old atlases because they reflect the view of the world at that time. I even purchased an old atlas from the 1920s. You'll never be able to replace that digitally because even if you scan all the maps, the feel of the old paper in my hand will be lost.
Even though I like my Kindle, there is something to be said for paper books now and in the future too.