Creating an ebook in EPUB format is easy,
for certain values of "easy". All one really needs 
is a text editor, a few command-line utilities; also needed is a working
knowledge of XHTML, CSS, along with an understanding of the format's
structure and required boilerplate. Creating
a well-formatted and attractive ebook is a bit harder. However, it can be
made easier with an application custom-made for the purpose. Sigil is an EPUB
editor that
provides the tooling authors and publishers may be looking for.

https://lwn.net/Articles/1054751/
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