Over at my Book Country interview, I talk a bit about the research behind worldbuilding, and the books I recommend. I did only give a few, I didn't want to go out of control, of course. That's what a personal blog is for, after all.
So, here's some of the books that have been helpful to me:
Guns, Germs & Steel by Jared Diamond.
Collapse, also by Diamond
These two books gives you a key primer on the core forces behind the rise and fall of civilizations and technology. Specifically, how the natural resources available, especially large domesticatable animals and key staple crops, are prime determining force for a successful civilization. Read these and you'll never have a city in the middle of barren wasteland on top of a mile-high mesa again.
The Omnivore’s Dilemma by Michael Pollan
This one gives you core concepts about production of food. While the chapters on industrial farming might not be as useful for the Fantasy Worldbuilder, the chapters on traditional farming and hunting/gathering definitely would be.
A History of the World in 6 Glasses by Tom Standage
Six drinks: beer, wine, spirits, coffee, tea and soda, and how each one represents a stage in history. Really crucial work for understanding, on some level, what each of these drinks are, where they come from, and what they mean culturally.
Country Cooking of France by Anne Willan
OK, this is a bit of an odd one, I'll admit, as it's actually a cookbook. But one of the key elements in this cookbook is the French concept of terroir, which I think is a fundamental to breaking down a nation into smaller cultural regions. Simply put, every culture will have its primary staples-- the main starches and proteins that form the backbone of their diet-- but then the finer details makes the dishes themselves. But the minor crops, herbs and spices that make the details can give all the difference: the "traditional dish" of five cities in the same country can each be, at its core, the same dish, while at the same time each dish can be unique to the character and flavor of that city.
Salt by Mark Kurlansky
Spice: A History of Temptation by Jack Turner
Both of these are similar books on the history of salt and spice, and the importance they played in culture, trade and exploration.
The Travels of Marco Polo by Marco Polo
Now, if you want some true source material on culture, trade and exploration, here is the holy grail. Polo's account is, of course, exaggerated in parts, but that also gives some insight into the nature of legends and mystery: what we think is on the other side of that ocean says as much about a culture as what is actually there.
The Disappearing Spoon by Sam Kean
There are probably several good books about science-as-history, but this one, focusing on the periodic table of the elements, is a personal favorite. For one, it might give you some ideas regarding the process of discovery and evolution of technology-- much of "science" up until a century ago was essentially the dabbling of the bored-- but also how basic elements are used.
I wish I had a good book to recommend about geography and mapmaking, since I feel that's such a crucial element of fantasy worldbuilding. Unfortunately, I don't have a key source. I have a fundamental knowledge from high school Honors Earth Science, and my own bits of specific research. But no general text to offer. Any suggestions?