Commoners weren't literate in those days, so I expect his literate customers would be wealthy and would be willing to pay for an illuminated book. But as books

Commoners weren't literate in those days, so I expect his literate customers would be wealthy and would be willing to pay for an illuminated book. But as books proliferated, it suddenly became advantageous for commoners to learn to read (the Bible was the typical primer used), and a virtuous cycle began and revolutionized everything.