Looks like a direct ripoff of an older version of Sigil's codebase with no attribution whatsoever to me. They didn't even change the names of the widgets. Real nice.
And for the record ... if you like Sigil with Book View, why not simply continue to use the older versions of Sigil with Book View? The versions you like aren't going to suddenly quit working. No need to pay someone else for the same older software that we're giving away.