When creating HTML or eBooks, my motto is the simpler the better.

If you are converting a fiction book, say a novel, you really don't need a bunch of different styles and fonts. Just keep it simple. If possible, start out with plain HTML that has all the styles and formatting stripped out of it. Do not use a program like MS Word to create HTML or RTF. You will get so much hardcoded formatting that you can never sort through all of it. It will be a huge mess. If possible just use plain tags without classes. Especially for a novel, all you really need to do is assign the basic text formatting in the BODY tag and let the ePub converter do the rest.

• So I suggest you use a plain and simple source file. For example a basic HTML file without much formatting. The less CSS formatting the better.
• Add your @font-face statements in the heading section of the HTML file.
• Add a reference to the @font-face statements in the BODY tag.
• Use calibre to convert the HTML file into an ePub file.

When creating your source file I would stay away from programs that add their own copious amounts of formatting. I suspect Sigil does, and I know MS Word does. Find a simple HTML editor like Homesite+ that does not mess with your HTML.