Originally Posted by kennyc
three is best, preferable on different types of storage media.
One type of storage medium is fine, so long as it's hard disk, IMHO. Today's hard disks are so big that the only feasible way (for the home user) to back them up is with other hard disks. I have a couple of external USB hard drives, one connected and one in reserve, and a script which automatically backs up my PC to the connected one every night. Every week I swap the backup one with the reserve one, and put it somewhere safe. That way I have three copies of everything that is over a week old, and two of everything less than a week but more than a day. The data less than a day isn't backed up, but as my main storage is RAID5, it's immune to single-disk failure in any case.
Ebooks are *so* small by today's standards that multiple backups really shouldn't be a problem.