Argthtjtr

This is actually a major consideration in modern building design.

Known as passive building, the idea is to minimise the ecological footprint of the building. Primarily by reducing the need for active temperature controls.

Simply following these principles in the construction of the building will greatly reduce any concerns he may have about the storage of the documents. However they're largely tied to modern principles and technology like insulation and double glazing.

He has some advantages though, the dry heat of a fire tends to reduce condensation in a way that central heating doesn't. The chimney of his building probably runs up through the space he's using for storage meaning that through the winter the room will be warmed by the fires below. A thatched roof will also offer greater temperature control than slate, reducing the range of temperatures the documents are exposed to.

Stone is good as thermal mass, but external stone tends to attract condensation on the inside, so it would be good for a chimney where it would hold heat for long periods, but not great for external walls where it would encourage mold. Better to use wattle and daub with its better insulating properties.

This is pretty easy and widely managed even today by some peoples cooling their homes in hot countries.

Work out where the prevailing wind comes from, build a ladder access floorless room on the roof which opens into the prevailing wind and funnels the air down into the house. If it gets too cold, close the door. If you get wind from a different direction like a seabreeze at certain times, have a another door. Normally you get a fairly steady dust free breeze and the higher your roof room the better.

If you just want to store your books, the hot room at the top is good as it is. Books burn at Fahrenheit 451 / 232 Celsius. Your Room would not get so hot. What kills books is mostly humidity, wich will be much less in a hot room at the top. And of course you must prevent exposition to sunlight, because that would destroy the ink on the paper.

If you really want to cool down a room, you can use two things:

• water and evaporation (but outside the room and you must save your books against humidity)


• Ice Blocks. Before electric fridges came up, Ice was cut in the winter and was used to cool already cool rooms (icebox). I do not know if this was already made in medieval ages, but it should be possible.

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.

This answer assumes you want to keep the books safe for generations and live in a warm climate.

According to the Wikipedia article on the conservation and restoration of books, manuscripts, documents and ephemera, there are the following elements that constitute to the degredation of paper et al. (highlighting is my own):

1. 
   Inherent vice
   
   

   
   

   Inherent vice is "the quality of a material or an object to self-destruct or to be unusually difficult to maintain". Paper, books, manuscripts, and ephemera are prime examples of materials subject to inherent vice.

   
   

   
   

The typical "old book smell" would be a classic example of chemicals and materials in the book that break down over time. Since these are primarily chemical reactions, cooling storage would be (as you pointed out) beneficial.

2. 
   Pest
   
   

   
   

   Insects and vermin are naturally attracted to paper because paper is made of cellulose, starch and protein, materials that provide sources of nourishment.
   [...]
   To best discourage infestation, a clean and dust-free environment is desirable: food and drink should be kept away from storage areas.

   
   

   
   

This may be seen in the form of blotches or other blemishes on pages, but can also be visible in other ways.

3. 
   Environmental conditions
   
   

   
   

   Extremes of temperature or relative humidity are damaging from either end of the spectrum (low or high).
   [...]

   
   
   

   Fluctuations in temperatures and humidity may also cause cockling: a wrinkling or puckering preventing the surface from laying flat.
   [...]

   
   
   

   Dust tends to absorb moisture, providing a suitable environment to attract mold growth and insects. Dust can also become acidic when combined with skin oils and the surface of paper.
   [...]

   
   
   

   All kinds of light (sunlight, artificial light, spotlights) can be harmful. Light can result in fading, darkening, bleaching, and cellulose breakdown. Some inks and other pigments will fade if exposed to light, especially ultraviolet (UV) light present in normal daylight and from fluorescent bulbs. [...] Minimal or no exposure to light is ideal.

   
   

   
   

Extreme temperatures and humidity can result in leather cracking, rotting and other nasty things.

Keeping humidity ideal will be difficult, as will keeping the environment stable and reducing dust.

Light won't be much of a problem for the books, since a hidden room seldom has windows and artificial light before electricity would be dangerous anyway, so you simply leave the room dark. The problem with light would be one's inability to see anything, making cleaning and managing storage, let alone reading, a challenging task.

Therefore we are left with the following conditions for the ideal room:

1. Cool (below ca. 20 C°/70 F°)




2. Dry-ish (30-50% humidity)




3. Clean (no dust, no life)




4. Dark (as little light as possible)

The first two we can measure using thermometers and hygrometers both of which would have been available before electricity (there are some really interesting designs, I would recommend doing a little research on the subject). You could have them both measuring the room while displaying the values outside of it, allowing 'remote' observation to regulate manually.

Self-regulating mechanical systems would be possible but quite probably unreliable and highly complex. You could perhaps construct an alerting system connected to the meters that makes a wound-up bell go of or something of the sorts, should the values become out-of-bounds.

To regulate temperature you could use many different methods as found on the site of the Permaculture Research Institute. Your ideal solution will depend on the climate of your region.

To regulate humidity you could use ventilation methods, causing water to evaporate so you can vent it via a skylight et al., or use absorbing materials such as charcoal or rock salt in large amount. Regulation would be done by adjusting the airflow or the amount/freshness of the absorbent material.

To keep your room clean you could seal your room as well as possible, reducing the risk of pests or dust entering the room, before killing everything off via your method of choice. This may include super-heating, freezing, suffocating (if the room is airtight) or using chemicals. If you decide to use chemicals, make sure you can remove all traces of it from the room to prevent damage to the books and poisoning yourself.

After you're sure everything is dead, do a spring clean of the room, making sure all parts of the room are easily accessible and as clean as possible. Move in your books after inspecting them for potential risks.

To prevent light I would probably build in a small, sealed 'window' that shines past the books. You can the use the ambient sunlight to do your work while keeping the amount of light as low as possible.

I hope this answer contains some inspiration for your final solution.