EPUB & PagePerfect™ eBooks are the standard format for eBooks in the NOOK store.
According to the accepted answer here:
When someone asks me what GUI to use I suggest Sigil. I find that Sigil, unlike it's alternatives, does not add unneeded code to your ePubs which cause bloating, errors, and slower viewing.
Its code is also on github.
Can I use Sigil to edit the text of the ePub file on NOOK of my computer of an book that I have created? I try editing an ePub in Sigil and it does not work on NOOK. If I create a simple ePub without DRM, is it readable on NOOK.