Unfortunately, you can't buy books from Amazon and place them on the Nook directly without stripping, except in the rare cases where a book is DRM-free (if the Product Details say "Simultaneous Device Usage: Unlimited", then you can get it and convert it without hassle; otherwise, it's only easily readable on a Kindle or Kindle app).
Amazon and the other brands use incompatible file formats; Kindle accepts Mobi and Topaz and non-DRM PDF, Nook uses ePub, PDF (with or without DRM), and eReader.
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