I use a combination of Sigil and Calibre for editing and creating ebooks - I work on my source text in Notepad, port it into Sigil for editing in .epub format (I like the fact that I can break the text into distinct chapters and work on shorter 'files' rather than bogging my slow laptop down with the entire text all at once), then export the .epub to Calibre and convert to whatever file format I need, as necessary.