This has become something of a fashion in paleographical and codicological studies these days. Scholarly tomes have been written on the subject and there is a website, Annotated Books Online, which provides digital facsimiles of mostly early printed books with handwritten annotations.
This is why I used the colophon from the Lindisfarne Gospels as an example of insular minuscule writing in Medieval Writing. The graphics for that exercise have now been upgraded courtesy of downloaded colour images from the British Library, where all pages of that historic work are now online.
So why did Aldred, scribe of the gloss, describe himself as an unworthy and most miserable priest? Was it a polite conventionality, or was he truly in awe of the mighty work which had been produced a couple of centuries before? Can you imagine him standing before the abbot jabbering, "What! You want me to write all over this! Are you kidding? Oh dear Lord I am so miserable and unworthy." OK, so I am getting a little imaginative here, but somehow it makes it all a little more human.