The open book in the foreground is about church marriage law.
Those books where written and copied from monks by hand. Which is for itself amazing enough, but if you have the chance to see such a old book in real life look at the details! You can find a lot of things the monks used to support their writing. Such as help lines. You might also find indices that two persons wrote on one page, which gives room for interpretations. And take a look at the initials - so much details in such a small "picture" - just amazing!