That's pretty much what epub files are. You can unzip them and then read the html they contain. You can also use something to strip the html tags and only plain text will remain.
Exactly. And some modern books are taking full advantage of that fact and including javascript for interactivity. Though I'm not really a fan of this direction, some of the childrens books I've worked with have been pretty cool.
Also note that while EPUBs are zips, the first few bytes are not compressed. Doesn't matter when unzipping, but if you want to make changes and re-zip you have to account for it.
