Welcome to the Nook world!
eBook purchased from eReader.com and created by dropbook display fine on Nook.
Regarding dictionary, so far you can't use other dictionary for either in-book lookup or standalone.
Atlantis word processing is another tool to create epub files, as easy as "save as ebook (epub)", from your existing MS Word documents.
https://www.mobileread.com/forums/showthread.php?t=48775