You can download Nook for PC and download the books and use Calibre to convert them. If you buy books with DRM, you will need to search for Apprentice Alf and Apprentice Harper for tools to remove the DRM. Or you can find DRM-free books (Tor books do not have DRM) and be able to take them straight into Calibre for conversion or use KindleGen to generate a mobi you can then use Send to Kindle to get to your Kindle. I've gotten a few Nook books because of gift cards and have been successful using Calibre to get them to a format for my Kindle.
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