Sigil is free. All sigil releases are here on github: https://github.com/Sigil-Ebook/Sigil/releases
You can also contribute to the project over there if you happen to be a developer.
For my Sigil tutorial on YouTube, i used version 0.9.2 and have not upgraded, yet. So, if you want my copy then go to https://github.com/Sigil-Ebook/Sigil/releases
Scroll down the page until you find my version 0.9.2 and download it for your operating system.
Converting books and comics to epub with sigil, the writen format: HERE
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