[RPG] I have a bunch of old books. How to find out how much they’re worth

Those books were licensed reprints done by a now-defunct third party company, Twenty-First Century Games, sometime in the late 1990s or early 2000s after TSR had been bought by Wizards of the Coast but before WotC began publishing their 3rd edition of D&D. They were created specifically for the collector's market, and it appears that their value has gone up since then. They are long since out of print.

Depending on the exact books and how many you have, these are worth a bit of pocket change compared to their original retail value. Noble Knight, an online retailer specialising in out-of-print roleplaying and wargaming products, has a section for Twenty-First Century Games's AD&D reprints that lists the miniature Dungeon Master's Guide at US$50, among others. Currently, eBay is listing a sale of the miniature Wilderness Survival Guide at 51 Euro (~US$64) or-best-offer (plus shipping from Italy), but that's probably not an accurate valuation of the book, as Noble Knight is selling the same for just US$15 plus shipping.

The value of your books individually is probably somewhere in there. Depending on the exact composition of the collection, it may also have additional value as a set to someone.

And therein lies the difficulty: you won't find anywhere to unload these for full value without a bit of work, and some skill in selling collector's items. The easy route would be to sell them to a reseller like Noble Knight, which will give you some fraction of their value but is relatively effortless; the hard route would be eBaying them or similar, as a set or individually, but only you can factor how much your time and the headache of shipping is worth to you.

I don't think this has given you a full, stand-alone answer that you can skip off to the bank with; but I hope it has given you the means and knowledge to move forward in the direction that most suits your resources and goals.

This is actually an intentional strategy on the part of WOTC. Obviously we don't have full insight into everything they are thinking, but Mearls did talk a good bit about the intended release schedule in the run up to the release of the edition.

Honestly, I think our current plan fits my ideal pretty well. It was also driven by the data we collected as part of the open playtest. We're looking at two major releases a year, with one or two books supporting each. Before I worked at WotC, I rarely used stuff beyond the core. (Mearls Reddit AMA)

So part of the reasoning in creating limited releases is that the feedback they got on the playtest indicated that their customers wanted limited releases.

At this point it looks like those releases won't be splat books, but will generally be drivers for their organized play events (The current event being ToD and the next one being a single book Elemental Evil campaign).

More importantly, the kinds of things you're looking for, will likely at least for starters, be published digitally, and take a long time to develop. For instance, if you were interested in Eberron, they have already published an Eberron playtest for 5e.

They've said in a few places (including the AMA linked above, here's a better digest), that they plan to release a lot of optional content, and potentially other things (like the setting materials you mention maybe) via their website rather than to do full publications for it.