I'll come out of lurker mode to add two books, which have been discussed a little elsewhere on here. They're both great at explaining complex societies, and so would probably help a lot when coming up with realistic cultures. Both are by Jared Diamond, and their overviews from wikipedia are:

Guns, Germs and Steel
The book attempts to explain why Eurasian civilizations (including North Africa) have survived and conquered others, while arguing against the idea that Eurasian hegemony is due to any form of Eurasian intellectual, moral or inherent genetic superiority. Diamond argues that the gaps in power and technology between human societies originate in environmental differences, which are amplified by various positive feedback loops. When cultural or genetic differences have favored Eurasians (for example, written language or the development among Eurasians of resistance to endemic diseases), he asserts that these advantages occurred because of the influence of geography on societies and cultures, and were not inherent in the Eurasian genomes.

Collapse
Collapse reviews the causes of historical and pre-historical incidences of societal collapse - particularly those involving significant influences from environmental changes, the effects of climate change, hostile neighbors, and trade partners - and considers the responses different societies have had to such threats. While the bulk of the book is concerned with the demise of these historical civilizations, Diamond also argues that humanity collectively faces, on a much larger scale, many of the same issues, with possibly catastrophic near-future consequences to many of the world's populations.