I'd be inclined to keep my reading and backup software separate, to avoid the Swiss Army Knife effect. Dropbox works quite well (here's
my referral link, if you want some extra space) for backups, but I tend to like doing my own offsite backups, otherwise known as burning my books to DVD and throwing it in a desk drawer somewhere. Since I just load my books on my Sony Reader, access isn't much of an issue. However, you might want to look into whether the calibre server can meet at least some of your needs.