I actually use 2 forms. I back up the back up.
I copy my books to a SD card. I'm looking at a 32 or a 64 gig one for book storage right now. I currently use a 8 gig but in my reader I use a 2 gig. I don't consider the 2 gig permanent. I use it as I do the drive on my ereader.
I also burn to disk.
Any of those can be corrupted and having a backup of the back up keeps it extra safe so I am not sitting there using what my mother use to say was "Those Words" when I get the "cannot read drive [or disk] " error.
I also learned the hard way that memory can go bad with age. I have some old floppy disks I pulled out to use for some old games and discovered on some I can't read half the disk.
Phil's rule: Make a backup of your backup.
And do NOT count what is on your computer as one of them.