Here is a link to the Calibre forum here on MR:
https://www.mobileread.com/forums/fo...play.php?f=166
I personally use it to manage my collection of ebooks. But I had a collection of epubs from before I moved to a Kindle and Calibre has been a fantastic tool to manage that collection and to "convert" my old epubs and load them onto my kindle.
If I only had Amazon books I am not sure how much value Calibre would add to me personally.