Practices of an Agile Developer by Venkat Subramaniam and Andy Hunt (Pragmatic Programmers)
With a plethora of software development methodologies and derivations available to developers today, it can be a daunting task trying to determine which fits best for a given shop. Even after a methodology is chosen, determining a concise set of standards for your group can be an even greater hurdle. Well, if your group has determined that test-driven, agile development is the best path, then look no further than Practices of an Agile Developer.
Building off of the Manifesto for Agile Software Development, it gives a succinct background to why agile development works, plus it details a solid roadmap for achieving your goals, while being highly entertaining. Practices of an Agile Developer describes the tools commonly used to aid in your development, along with the theories and mindsets behind a successful TDD shop. If you are looking at incorporating agile development into your development cycle, this should be your first reference.