In addition to constructing the building in a certain way, he could seal the books in thick-walled clay pots.

A good starting point is a pot or amphora with a lid made of burned clay. Place the books inside and seal the lid with fresh clay (of course you cannot burn it with the books inside).

The clay evens out changes in temperature and humidity and keeps insects out that might otherwise destroy the books. They do not work like an air condition, but they should protect the books better than lying in open shelves.

If you need to access the books inside, you carefully scratch the seal of unburned clay away from the lid. Due to the difference in hardness between burned ans unburned clay, the both should be easy enough to seperate.

The risk of this method is that moisture in the pots (like from a leaking roof) causes the books to rot (or get mouldy) faster than without a pot because they take longer to dry.

In addition to constructing the building in a certain way, he could seal the books in thick-walled clay pots.

A good starting point is a pot or amphora with a lid made of burned clay. Place the books inside and seal the lid with fresh clay (of course you cannot burn it with the books inside).

The clay evens out changes in temperature and humidity and keeps insects out that might otherwise destroy the books. They do not work like an air condition, but they should protect the books better than lying in open shelves.

If you need to access the books inside, you carefully scratch the seal of unburned clay away from the lid. Due to the difference in hardness between burned and unburned clay, they should be easy enough to seperate.

The risk of this method is that moisture in the pots (like from a leaking roof) causes the books to rot (or get mouldy) faster than without a pot because they take longer to dry.