There are many interesting stories about it, since it is said to have been created by one single man. There is a legend that it was a monk who sold his soul to the devil and created this piece in one night after doing so. Much of this has to do with the huge page donning an image of the devil.
It's an utterly fascinating book. I urge you to watch this National Geographic piece on it if you are at all interested in manuscript arts. You can also browse images of the manuscript at the National Library of Sweden.