Please consider the Sigil User Forums at Mobileread to be the primary resource for getting help with your Sigil questions. There’s lots of knowledgeable users there to help, and the developers monitor the threads/posts there as well. Please consider joining the forum (and the Sigil discussions).

The Sigil wiki can be found here. Many important support links and much relevant information can be found there.

The latest release of the Sigil user guide can always be found in its own repository (in EPUB format), as well as in the docs directory of the source tree.