What climate, temperature, etc. issues do I need to build a "good enough" storage shed for my books?

IANALibrarian, but I think you'd be better off storing them in the house if that's at all possible. Optimum conditions for books (within reason) won't be much different from optimum conditions for you, meaning you'll need to moderate temperature and humidity, requiring insulation, power, etc. Climate control in a small outbuilding will probably be less efficient than it is in the house, if for no other reason than the ratio of wall/roof area to enclosed volume is much larger.

Sheds have one major advantage over houses - You don't need windows. So, IF you build one with normal-scale insulated walls (instead of the standard one-sided 2x4-framed walls you normally see used for sheds), it'll cost almost nothing to heat/air condition. Hell, for that matter, I'd suggest reading up on DIY icehouses - Although you don't want to keep them frozen, you do have similar requirements as far as isolating the inside from the outside in terms of temperature and humidity. As an alternative, however - Have you considered buying a seriously EOL RV? One that you can get to your house but wouldn't trust to live through even a 20 mile road trip? Rip out the entire interior, line it with bookshelves, get an aftermarket rooftop HVAC unit, and bam, you have a climate controlled library (that might actually count as mobile - You could at least tow it) for cheaper than a 12x12 shed kit from Home Depot.

Extending pla's answer; consider an old library van? Many countries have or had them for (mainly) their rural or island services. I'm not too up on the specifics of buying them, tho' aware that there was an ex-library van convoy at a library conference (not sure where) very roughly five years or so ago. So they exist. http://www.flickr.com/photos/silversprite/sets/72057594102516175/with/76260874/ which visited us every three weeks, when we lived on a tiny Scottish island for several years in the last decade. Advantage: it's already kitted out with shelving and concealed storage space.

My house actually is a 8ft x 20ft RV, so no, there is no room for them in them in the "house" and the final dimensions of the storage "shed" may actually be bigger than that of the "house."

I am a librarian, though I'll also say many of us (including me) have no training in things like how to build free-standing libraries. What I do know: the humidity level is what's really important. If the humidity is too high, you'll get mold and lots of it. I think you want humidity of 50% or less. If you aren't going to include any kind of a/c for summer, that could be a real problem. Try googling something like humidity level books. That got me this page from the http://libraries.ucsd.edu/preservation/collectionprespolicytempRH.html

Perhaps you could buy a used shipping container similar to this: http://www.ebay.com/itm/Shipping-container-or-storage-cargo-40-ft-feet-Grade-A-sea-worthy-Good-quality-/200700437829?pt=LH_DefaultDomain_0&hash=item2ebaada545 I'm guessing these are available locally almost anywhere in the country. To keeps the books happy you could run a de-humidifier in the container to keep the humidity down. IANAL, but I would imagine that books won't be harmed by the normal temperature range of your climate.

According to http://swingleydev.com/blog/?p=358, books run about 12 per linear foot. So 1200 books would be about 100 linear feet of shelving. That's a lot in a living room (and even more in a small RV), but isn't much in a shed. How big of an issue is cost? Ignoring budget, the nicest option would be to just build a shed/detached garage/outbuilding (the specific name might have permitting implications, so ask if one is cheaper to permit), insulated as well as you can afford, and with an HVAC system sized for the building (a simple wall unit would probably be enough). That will keep the temperature relatively constant and the summer humidity under control. But that implies some real costs -- at least 2x4 and probably 2x6 walls with insulation, good doors/windows, vapor barriers, electricity and maybe gas, etc. Controlling humidity is the real key, because damp books get mildew and mold very quickly.

My biggest concern with this plan wouldn't be the issue of best-preservation (which is important but solvable) but of failure mode. You are, very literally, putting all of your eggs in a single basket. I would worry way more about what happens if there is any failure in the structure that led to water leakage, which is the most likely way for this to go wrong. The http://www.loc.gov/preservation/care/books.html, with links out to good, solid resources and a bibliography attached, if you still want to go this route. Me? I'd be thinking about digitization and not worrying quite as much about the physical instantiation of them. If you do move forward with this, I would vastly overbuild the structure as well. Books are _heavy_, with weight-bearing loads on the floors of more than you might expect. The recommendation for tightly-organized library stacks is something like 300 pounds per square foot.

I was watching something on television a couple of days ago about how the Science Museum in the UK was building climate-controlled storage for their transport collection (old cars, bicycles, carriages etc.) Their solution was to build storage sheds using http://en.wikipedia.org/wiki/Hempcrete panels, which they found was excellent for controlling temperature and humidity, and thus preserving the more delicate materials (fabrics, leather and so on).