I saw a "systemless" nonfiction book on Narnia back in junior-high school, and a collection of fantasy maps had one of of Lewis's maps for the settings which I assume can be found there or somewhere else. Most major settings do have some book that gives a nonfiction overview of the setting, and not every RPG book can or does give all the political details and everything on every settlement you might want to play in.
Frankly, I don't think I'd jump at buying setting-free books for gaming. For one, one of the questions that makes it hard to stat up a setting is the stats for an orc/klingon/kziniti; how strong is it, how intelligent, how tall and heavy (and what's the range on that), etc. Just about any system will give me that information in a form I can translate to my favorite system. If I have D&D stats, I can at least make a guess at how to translate them to GURPS. It at very least should make sure that stuff like usable claws and infravision is noted.
I also agree with the concept that the system matters. In D&D, high-level characters are superheros and can be challenged mainly by other high-level characters and monsters. If a high-level character was still a mortal human, and an army could still drop them, and was the only thing that could stop a dragon or other major monster, it'd make for an entirely different world. Even minor changes, like making the first spell that could do force damage 3rd level instead of 1st could make large differences in the power of certain creatures.
I suspect that Freeport will still be D&D, just not officially. To which I can only see not have the characters stated up as a disadvantage; if I'm forced to assume something equivalent to the D&D magic system, it'll be easier for me to have some sort of D&Dish stats there.