Know thy enemy
Your books worst enemy is of course, humidity. Humidity is also conditio sine qua non for the dreaded mold that will happily munch on the cellulose leaving behind not much in the way of a cultural legacy for future humans enslaved by Skynet. The details provided on your question indicate that temperature may not be a concern, since it falls within the range recommended by the Northwest Documentation Center, and endorsed by the Library of Congress. These resources also point to an ideal relative air humidity between 30% and 50%.
However, the most important factor is the stability of the environment rather than the actual specs chosen for such environment, as rapid changes in environmental conditions are the main culprit of document deterioration, as the rate of most chemical reactions affecting cellulose substrates is doubled with every 10°C of temperature increase. High temperature combined with a high humidity concentration will encourage insect activity and mold growth, in addition to chemical deterioration.
Chose your battle wisely.
Since the temperature problem appears to be out of your control, and the temperature variations due to seasonal changes fall within the range of the above recommendations, my advice is to focus your efforts on the variable you can control vis-a-vis: humidity.
Size up the monster.
Peace of mind regarding the environmental conditions of your collection can only be achieved by properly measuring this conditions. You may wish to acquire a hygrometer which may not be as expensive an instrument as one may think, (around USD$ 10.00 at the time of writing) and can be considered chump change compared to the value of your collection.
In order for the hygrometer to be effective you may wish to arrange your collection within an enclosed bookshelf, with the books and documents tightly packed as space allows, per the recommendations cited above.
The idea is to avoid controlling the humidity of your entire apartment, and be able to measure humidity conditions within the critical document storage environment. It doesent need to be airtight, just restrict the airflow enough to keep humidity out and optimize the performance of any dissecant or hygroscopic material you choose to control the environment within.
Select your ideal arsenal for battle
To get rid of humidity, and thus making the acquisition of an expensive fungicide a matter of redundancy, enclose a canister of a highly hygroscopic substance along with your dead-tree books collection, once this is done, any humidity within your bookshelf ends up getting sucked and sequestered by such substance, leaving a nice, dry, mold-unfriendly measured environment once you close the lids.
For this purpose, I recommend the use of silica gel packets that come with most electronic device or apparel packaging. However, since your collection seems rather extended, this alternative may prove impractical, unless you've been collecting the silica gel on your purchases for the last few years.
These can be purchased in bulk, (around USD$ 5.00 for 100 1g packets on Amazon, at the time of writing) and have the advantage of coming in an unobtrusive, small paper package that will allow moisture in, but won't let the silica gel spread all over.
A cheap alternative is to roll your own. One of the most hygroscopic substances available in the cheap is a mineral called zeolite which is the main ingredient of most brands of commercial non-clumping cat litter sand.
Just build your canister for zeolite using an average coffee filter for the purpose. It's porous enough to keep the zeolite in the envelope while not diminishing it's affinity for water in any discernable way.
Wait!, how can i make sure i don't over-do it?
Glad you asked, since future Skynet enslaved humans will be very disappointed if their hope for cultural legacy turns to dry dust in their hands upon touching it.
The question here is how much desiccant is about right to keep relative humidity between 30% and 50% The quick answer is: trial and error. That's why we're sizing up the monster and measuring the environment with the hygrometer we wisely invested in. Most silica gel manufacturers recommend this approach, but if we're sparing no expense, then purchasing desiccant canisters certified conformant to military spec MIL-D-3464E is the way to go.
Be advised, though, that MIL-SPEC certified materials will cost anywhere between 200% to 500% more than your garden variety non-certified consumer grade materials. What you're buying with that cost difference is the certainty that the material will perform as stated on the specification with minimal deviation (again, as allowed by the same spec)
The specification defines a unit of dissecant, as the amount of material that will absorb 3g of water at 20% Relative humidity and 6g of water at 40% relative humidity, for bulk material; and 2.85g and 5.70g of water respectively for bagged materials.
Thus you can calculate the amount of dissecant units required given your environmental conditions. Note however that how much material is in a unit of dissecant depends on the specific materal, for military grade silica gel, a unit is about 50g in a 4.5" x 3" sealed porous paper bag
Or just use this handy calculator
OMG! TMI! TL;DR!
- Get a hygrometer
- Restrict airflow to your collection (pull a plastic cover over your bookshelf)
- Place a canister of silica gel or zeolite within the bookshelf
- Keep books tightly packed
- Monitor the humidity on the bookshelf, adding silica gel as required
- Periodically "regenerate" the silica gel per manufacturer instructions
- Just toss a silica gel baggie on your zip-loc enclosed documents, and change from time to time
- Don't feed skynet! Stop training it's algorithms by avoiding the use of Facebook like the plague (kidding)