I didn´t know what Sigil was

Sigil

Windows, GNU/Linux, Mac OS X

If you read the previous post you notice that they mention Sigil as a "needed" tool. I didn´t know what Sigil was. But it is free... Yes !! Free, Open source under GPLv3. Currently the only application that can also open and edit EPUB books, instead of just converting from other formats to EPUB. Does not currently support embedding video or audio in EPUB.