I work on a Macintosh. I render an original file(s) as plain text and work in TextEdit. I do a find and replace on carriage returns with <p></p>. I open this file in Sigil and work on it from there.
Although Apple Pages word processor has an epub export, I am not familiar enough with it to recommend it. I have and ocassinally use TextWrangler, but it's like using heavy construction equipment when I need garden tools.