It's best to regularly perform a full back up of your computer and make sure that the Blurb folder containing your books is part of that back up. That way, if your computer crashes you'll have a back up of your book projects (and the rest of the files on your computer, too!)

Otherwise, you can back up book files individually by locating the book file (search for files ending in ".blurb") and copying that .blurb file to a hard drive, a cloud back up or other back up source.