With Epub, or other ebook formats, one can take advantage of the format and do things that are difficult to do in a paper book. Hyperlinking, extra diagrams, more page breaks - what else? Most books for kindle I can find on amazon have been "converted" from paper books, but have identical content to their paper counterparts. Is there a good example of a popular/good chess book that is much easier to study on electronic device?
It depends on what you mean by an "electronic device", but there has been ebook versions of chess books for quite some time, from publishers like Everyman, although admittedly these are nothing like regular ebooks. Of course your device must be capable of running CB reader or an equivalent that can open the chessbase format files.
On the more practical side of things, chess books don't sell very well and formatting even the printed copy is a lot of work. QualityChess has no intention of producing ebooks that can be easily copied:
Q: Will your books be available as pgn or ChessBase e-books?
A: As long as it is easy to copy these forms of readers, it makes absolutely no sense. Why other companies choose to do this is a mystery.
I'm unaware of any special epub books. The problem with epub is that you can't move the pieces on the boards.
Most chess book publishers seem to be going with separate apps: Everyman's Chess Viewer reads their pgn books, and the rest use proprietary formats that display a chess board where you can play through the game by clicking on the notation or arrows: e+chess/New In Chess Books and Files (iOS only?), Gambit Publications' Chess Studio, Forward Chess (Quality Chess, New In Chess, Informant, Chess Stars), Chessapps, MChess and Smartchess. Chessapps and Smartchess give you access to an engine for analyzing the positions. Chessapps also lets you save your notes.
I haven't tried it, but there is an app called 'Cbh to pgn' for converting on Android.