My personal preferences in order are HTML, RTF, and TXT. All other formats are specialized to a particular reader (LIT, AZW, LRF, PDB, etc.). I avoid these unless left with no other choice.
Epub's are HTML files zipped up in a package including images and metadata, like a web site in one file.
So HTML's convert quickly to epub. RTF and TXT are also quick to convert but need some formatting as they convert but usually convert with no problems.
If you learn a little HTML coding you can reformat your book before it is added to Calibre if you wish or you can use Sigil to turn it into an epub yourself so it looks like you like for your Nook.
Happy Tuesday
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