are you buying so many books that you need an automated process ???
manually:
install the de drm plugin for calibre - can't be explained here but you'll find hints on here for what to google.
connect kindle to our PC on USB,
open kindle device as a drive on PC, navigate to documents folder. your bought books will unfortunately have gibberish names but drag & drop them into calibre main window & than calibre will de-drm and add them to library. it willl I think get title, author etc from within the book so you'll have sensible entries within calibre
if you miss out the DeDRm stage, calibre can still store backup copies but cannot edit them / cannot display them in viewer & I'm not sure how/if it gets hold of correct titles etc
|