Peachpit Press's eBook Deal of the Week, for this week, is EPUB Straight to the Point: Creating ebooks for the Apple iPad and other ereaders, by Elizabeth Castro. This book is recommended. It has a great introduction to EPUB as well as workflows for creating EPUBs from Microsoft Word and Adobe InDesign.
Normally $23.99 it is on sale this week only for $9.99.
I strongly recommend checking this out if you are interested in producing books, documentation, course readers or other eBook type content in EPUB format. It does not cover the Sigil workflow, should you choose to go that route. But, there is still plenty of useful information here, especially at that price point.