Aliens and Alien Societies
Stanley Schmidt, edited by Ben Bova
People often ask where to start reading up on factors to consider in worldbuilding, and I think this book should be up there with the advice to use sites like Wikipedia.
In each chapter, it goes through a different aspects of your fictional world, with factors to consider when you want your setting to be believable and you want to avoid falling into the traps of incorrect cliches.
