Clintbradford has some interesting points. To be absolutely shure, you should have at least 3 copies of your ebooks. One on the computer, one back-up on an external storage media, such as an archival grade DVD-R, and one off-location backup, such as at your mom's house, who lives far away (in case your house burns down). You might also consider placing a backup in a safety-deposit box in a bank.
If you are backing up Adobe Digital Editions files, make sure you copy them from your "Documents / My Digital Editions" folder. DO NOT backup the .acsm file.
Chosing the right storage medium might be a hard task, but a reputable USB flash drive or archival grade CD/DVD media should do the trick. One example:
https://www.blankmedia.ca/proddetail3.asp?id=1114