A question...I now have a large number of books on my Kin...

A question...
I now have a large number of books on my Kindle. Can I move them over onto my computer, so that, if my Kindle gives up the ghost and I have to get a new one, I can transfer them over to the new one?
If so, how do I do this? (I've tried drag-and-drop but the computer claims it can't read the files - each book seems to consist of three files).
All and any advice will be appreciated.
Many thanks.