title_body: Can books be transferred from Nooks to Kindles and back?
upvoted_answer: You may want to try using the Calibre Ebook Manager with the deDRM plugin to get the ebook files, sans-DRM, and then move them onto the Kindle. It works, but it won't sync them or automatically move newly purchased books; each book must be done individually. 
  The Digital Reader has some helpful tutorials on how to do this:
  
  To strip DRM from Kindle ebooks
  
  To strip DRM from Nook ebooks
  
  Note that DRM removal is illegal in some countries. Please read and follow your local copyright laws.