The Apple Developer Connection just posted Working with Xcode Build Settings. It builds on Understanding Xcode Projects:
There is a lot of flexibility in the Xcode build system for customization, so you can choose to keep your settings simple or make them as complex as you like. But for most purposes, a basic understanding of how build settings and build configurations work will make this topic easy to master—once you have these concepts clear, you can choose the settings that work best for you.
If you’re developing Mac OS X software with Xcode, read Understanding Xcode Projects and Apple Developer Connection posted Working with Xcode Build Settings