Welcome to MobileRead.
I had a scary situation a while back when my computer crashed, and I had NOTHING backed up.
Was able to recover Calibre (over 900 books) and pictures, but it taught me a lesson.
I now do it two ways...........Carbonite, which is super simple and backs up for you, and an external hard drive, using
http://www.handybackup.com/
This way, no matter what happens, I should be able to recover my books.
www.dropbox.com is another free file sharing program which allows you to share books, pictures, etc, but also backs up whatever you put there. You get a certain amount of space free, and when you invite someone to 'share' your file, and they accept, your space is increased.