typically defined as books sold in electronic format - typically in a closed binary form
see Information Ecosystem Roadmap
online "markets" - esp good for Self Publishing
a special case has been those designed for dedicated hardware (e.g. Kindle)
HyperCard was an early tool for authoring these
crosses over with HyperText books like those made with StorySpace
http://drinkmalk.com/ - for commercial titles you already own
Daniel Jordi just goes ahead and scans and OCRs the books he buys/likes.
Some are in the Public Domain outside the US because the author died more than 50 years ago.
Barnes And Noble stopped selling them in 2003 after launching that store in 2000. Then announced their "nook" hardware in 2009.
Will Text-Book-s go this direction?
Does Wikipedia count?