The Metz Pontifical, also called The Pontifical of Renaud de Bar, an illuminated manuscript produced for Renaut de Bar, Bishop of Metz (1303-1316), and preserved at the Fitzwilliam Museum, Cambridge, has the added virtue, from the standpoint of historical research, of being unfinished. Work on the manuscript stopped in 1316 upon the death of Bishop Renaut de Bar, and its unfinished state shows the evolution of images; from drawings, through under-painting and gilding to finished images. Its manner of production is shown in an interesting flash animation on the Fitzwilliam website at this link. The manuscript was donated to the Fitzwilliam Museum by Henry Yates Thompson through the auspices of the museum's director, Sydney Cockerell.
Fifty-two other illuminated manuscripts owned by Yates Thompson are preserved in the British Library, which has an extensive article about Thompson and his fabulous collection at this link.