Lot of posters aren't taking into account one really pertinent point. At that time, faith was absolute. There was a God, there was a christ, and the holy spirit was more than just the 3rd figurehead. These beliefs were set in stone. It was on the order of perjury of their immortal soul to deface a religious manuscript.

As for her existence, it really isn't that far fetched for a noblewoman to enter the service of the church at some point, and less surprising that they were literate in the local Olde English dialect and Latin. If her father was unable to find a match, or she was intractable, the Abbess position of a religious order would be the equivalent to a corporate golden parachute.

I'm guessing the book was hers, personally, and likely that was what she did while listening to claimants. Essentially, doodling. Why she didn't leave a family name, likely because she was known, and her name was enough.