I provide you the following chat conversation I had with an Amazon representative:
You are now connected to Katherine from Amazon.com
Me:I bought a DRM-free Kindle book and I'd like to download it and put it on my Nook. However, the "Actions" button doesn't seem to have a "download" option any longer.
Katherine:Hello, thank you for contacting Kindle Technical Support. My name is Katherine. I understand you would like to download a book to your Nook device.
Well, download to my computer and transfer to my Nook
Katherine:I understand Eric, however you are contacting the Kindle support department, in this case, if you have issues or need to download the book, first you need to contact them for further assistance. First you need to know if the book you have is support it on the Nook and then they can definitely help you to download the book
Me:I'm not sure I understand.
It's a DRM-free Kindle book
I want to download it, change it into an ePub using Calibre and load it onto my Nook
But I can't find a way to get the source .Mobi file
I used to go to "Manage My Kindle" click "Actions..." and click "Download"
But that option no longer seems to exist
Katherine: I am very sorry Eric, once again I can help you downloading books to Kindles, not Nook. If you are having issues now with it, it will be better if you can contact them. You can read content you buy from the Kindle Store (such as books, newspapers, magazines, and blogs) on most Kindle devices or Kindle reading apps registered to your Amazon.com account.
Me:This has nothing to do with Nook. The problem would be the same if I wanted to download all my .mobi files to put on a personal hard drive for back-up.
Katherine:You might find more information in this link
http://www.barnesandnoble.com/u/NOOK-Su...(See full link)
I understand is not related with Nook, I can't just tell you that our books are supported for Kindle. If you were able to download to the Nook in the past, and this time is not working, could be that specific file is not supported, is better for you to ask them if in this case you can convert it or change the file to download it