In Sigil, Help/ F1 is labelled "User Guide".
It takes me to
https://github.com/user-none/Sigil/tree/master/docs
where you can download an epub version of the 0.72 User Guide. You can't view it.
It should go to
http://web.sigil.googlecode.com/git/index.html where there is an online version of the same manual.