Use Calibre to convert the ePub file to MOBI or even KF8/AZW3 which the Kindle can read natively. The interface takes some time to acclimate to, but the program is free and also about the only game in town. I routinely use it to sideload books I get from sources other than Amazon. Keep in mind Calibre only works on books without any kind of DRM. Not sure if the ebooks you get from B&N have any kind of DRM added on or not and if so, you're on your own to figure out how to deal with that.
http://calibre-ebook.com/
Hello fellow readers.
There must be a way to buy a Nook book - without owning the device - and be able to put it safely on my Kindle Paperwhite. Doesn't Nook or Barnes and Noble have a library where the books of each customer are stored online? Kindle allows one to "manage" one's ebooks.
P.S. I do not own a Kindle Fire. This is strictly Paperwhite.
Thanks.

Chowhound
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