A comprehensive manual on building viable fantasy worlds. Extremely well-detailed from cosmological down to social questions. The publisher has also released portions of the book as smaller pdfs for a more focused approach.
The only complaint I can amke is that the realistic approach precludes a discussion of odd, magic-high worlds like Discworld. For much the same reason it doesn't high-concept fantasy ideas like underground seas or dust oceans. But if you are looking for logical, real-world principles this is the best you can get.
I also want to compliment the publisher's customer service, which was top-notch.
A Magical Society: Ecology and Culture