Some US/Korean (I think) firm has created ViewPorter, a creator/editor application for epub 2 and 3. It is presently in beta, and has some (a lot, actually!) glitches - don't use it for anything serious! - it's "free until version 1.0", no price estimation is given.
Those of us used to Sigil will recognise quite a lot in this piece of software, but the creators doesn't seem to credit the Sigil programmers, even if it is obvious, where some of the parts, if not all, of the program comes from (It may be legal, since Sigil is opensource, but nevertheless
)
Anyway, the site to get the program from is:
http://www.viewporter.com/
(and fyi: I am in no way whatsoever affiliated with Viewporter or OrangeDigit!)
Regards,
Kim