I'm surprised it's only 20% to be honest
Book publishers are in the same position the music industry was 10 years ago, and if they don't watch it they'll get into the same mess.
Basically we have a few different, incompatible standards, with Amazon Kindle, Epub and DRM protected Epub. This is pretty much the situation we had with music, where MP3, Apple AAC and Sony Atrac all competed, and just caused confusion and directly led to people just downloading unprotected MP3 files that could be played on any player.
Many ebooks are more expensive than paperbacks, this is partly down to VAT, but not all.
The we get down to the old, what to do once you've read the book problem. With paper books you can sell them, give them away, or lend them to a friend or colleague. But with ebooks that's generally not possible, although Amazon are trialling a system where you can lend a book for two weeks, and during that period you can't open the book on your devices.
Some libraries allow you to borrow ebooks as well, I've not experienced this and would be interesting in seeing how it works...