For one thing, any guide that mentions Book View is referring to older versions of Sigil (just looked at your link and it's from 2014). I seem to remember that Book View was removed 6 years ago and replaced by Page Edit after Sigil 0.9.14.
If you only have one HTML file, just right click on it and follow Link Stylesheets.
Perhaps I should ask what version of Sigil you are using? Hopefully, 2.4.2 at least.
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