Awesome! Looking nice so far - loaded my largest epub, a few minutes of load time and it's usable, memory use ok and saving fine.
The spell checker, there's no option to actually disable it. Selecting 'None' as the dict seems to still use an empty dictionary, after restarting with 'None' selected, it defaulted to the German (first item) on the next launch. I think a button in the checking toolbar to toggle spellcheck might also be handy. I also didn't expect it to be used only in the code view, not that it's a huge problem, but it'd have thought it would be the book view (some people might not find it).
The spell check does not seem to deal with 'fancy' quotation marks, it will happily correct them to the boring version should you wish. I'm sure that could be fixed in the dict directly, but seems a bit odd (seems it might be a limitation of hspell tho).
The toolbar with validation/tidy... is called "Quick Settings", tho the menu is "Tools", seems a bit odd to call them settings.
The view mode does not apply for the session, swapping to an unopened file will default to the active view mode (i.e code view if I am active in the code panel of a split view). Tho I think this is nothing new, it's always seemed a bit odd that it doesn't just use the same layout.
Anyhoo, bed time. :/