For basic "proofreading" - checking that my "i"s are dotted and "t's" crossed - I just use Sigil. That way I can change any mistakes I see right away. I also proof it in code view, that way I don't run across any of the issues.
For checking the "formatting" I use ADE and Kindle Previewer and as many iOS apps as I can get to load an ePub.
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