If you stick to open, non DRM formats, there is no reason you can't read your books again, no matter the device you choose.
People have already done it with music in digital format, text is even easier. The epub format for example.
There are many ereaders without any sort of connectivity, no wifi or any other nonsense, just an usb cable and its recognized as a simple USB mass device (like a thumb drive), and some even take flash cards or such. Most of these can read the open formats perfectly.
The ereader is not an electronic book. It is an electronic LIBRARY. There is just no comparison. People often imagine themselves carrying a book, or the device... But the device is not a single book, it is tons of them, BUILDINGS of them. I mean, come on, a typical book is about 5mb, and the "pocket" ereader with 5" pearl white eink screen i use comes with 2gb of storage...
Backup? The same as with any other 2gb thumb drive.
Carrying more than 3 physical books is problematic, let alone thousands of them. You can now have the complete works ever written by somebody or from a subject. Also the device often allows you to search, just type the word or phrase and there it is!
Physical books can last a lot but they can also deteriorate, especially with public use and abuse; and, often works are never published again. With digital, nothing needs to be lost ever.
Remember, one of the oldest libraries was burnt down by American troops in Baghdad, the oldest known remaining human writings were lost forever and only digital pictures remain... You can go back in history to find again and again how libraries and writings of all forms were burnt down and lost.
Books are nice but fragile, and heavy, and impractical and time consuming to reproduce, and prone to idiots burning them. And in the rare situation you needed one, you can always print it back to deadtree format. So, various loads of trucks when you move, or a small and compact ereader?
The ereader might not be the answer for bookshops (unless they learn to sell books without DRM, like some did with mp3); but the age of libraries in the hands of everyone is already here.
Of course obsolete business models and copyright law might not stand it, but the fact won't change, the genie is out.
As for libraries, they have a limited number of copies, and limited working hours. IMO they should dedicate themselves to preserve physical books in controlled conditions and make sure there is a digital copy of everything, correct mistakes etc. People would no longer need a library card, just give everyone a reader with the whole thing, or at least a thumb drive or let people bring their own so they copy all they want to read at home.
Sooner than later all libraries should sync with each other, and people with them. There is no reason some have books others don't, at least not in digital form, yes, all languages, all subjects.
This is mundane compared to the stuff being done already with video and music.