I don't know if you've seen this news yet, but the online GMail service has been losing emails from some users' inbox this weekend. Actually, it would be more accurate to say everything has disappeared from some GMail accounts. <-If that doesn't scare you, it should.
Google are working on a fix, but I wouldn't recommend taking a chance. It hasn't happened to me, but I read about this problem yesterday and backed up everything with Thunderbird just in case. You really should do the same.
While we're on the topic, when was the last time you backed up your ebook collection? You can't rely on the ebookstore being there for you to download again. Fictionwise, Sony, and Amazon have all pulled ebooks at one time or another, and I know of at least 1 ebookstore that closed without warning.
Unfortunately, a lot of ebooks are encumbered by DRM, and that DRM will screw up the backup process. Now might be a good time to start learning about how to strip DRM so you can protect your purchases.