This is a book of canon law by Pope Clement V (around 1264 to 1314). The original text is surrounded by the commentary of Giovanni d’Andrea (around 1270 to 1348), the most renowned expert on the subject in the late Middle Ages. It's an example of an incunabulum, a text printed before 1501.
The book was printed by Peter Schöffer, an apprentice to Johannes Gutenberg and who successfully took over his press. The technological advances pioneered by Johannes Gutenberg revolutionised printing.
Many early printers did not wish to stray far from established forms, especially the aesthetics of luxurious manuscript books. They often imitated the style of handwritten books that readers were already familiar with.
The page contained spaces for painted and illuminated letters to be added. These would be decorated by hand using gold, silver, or coloured designs. This allowed the printers to mass produce a volume that would also be lavishly decorated.
[Library reference: Inc.1.7]