I have always struggled to understand the fundamental book about 13th-century Parisian illumination: Robert Branner,
Manuscript Painting in Paris during the Reign of Saint Louis: A Study of Styles (University of California Press, 1977).
I have had to grapple with it again in earnest during the past couple of years, while cataloguing leaves in the McCarthy Collection, several of which have been attributed (wrongly, in my opinion) to artists and ateliers defined and named by Branner, including "The Dominican Painter", "The Leber Group", "The Atelier of the Vienna Moralized Bibles", and "The Johannes Grusch Atelier".
The latter atelier has been the subject of an extended
exchange on Twitter this week, and in the course of trying to understand Branner's definition of the style(s), I went looking for digitized versions of the manuscripts he cites. One of them is a Bible at the Free Library, Philadelphia (MS Lewis E 242), recently
digitized as part of the Bibliophilly project.