Illuminators & Illustrators
The British Library Guide to Manuscript Illumination: History and Techniques

Medieval Illuminators and Their Methods of Work

The Alchemy of Paint: Art, Science and Secrets from the Middle Ages

Additional links on the subject of medieval manuscript illumination here.

Medieval Illuminators and Their Methods of Work indicates that these illustrations in a mid-13th century Bible are of a scribe and an illuminator at work, but I'm not sure that he’s painting in a book, rather than on a panel (like these painters).