Sigil is available for Linux, too, though I have used it on Windows only. It’s free, simple to use, and can be used both in Wysiwyg mode and in HTML mode, though the wysiwyg mode is rather limited in functionality. It would probably be sufficient for your purposes.
Sigil does not read .doc or .rtf files, but you can open such files in your word processor and save as HTML, then open the HTML file in Sigil.
Caveat: What Sigil shows in wysiwyg view is just one view. Different e-book readers display .epub files differently. But Sigil gives a reasonably realistic view.
(I’m somewhat biased. I’ve provided the localization data for Finnish in Sigil, and I’ve written an e-book, in Finnish, about writing e-books in Sigil.)
