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):
- 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.
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.
- 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:
Cool (below ca. 20 C°/70 F°)
Dry-ish (30-50% humidity)
Clean (no dust, no life)
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.