A couple of years ago I got to read Martin Fowler's fascinating paper on "The New Methodology". That paper/very short book summarizes the approaches taken by Agile development methodologies, how the came to be, and what the differences between Agile and "Engineering Methodologies" usually are. that paper gave me a new outlook on concepts I thought I had grasped before, giving more depth and understanding of the "why" and "how it came to be" parts of things I firmly believed and believe today, and not a week goes by that I don't talk about some of the things I've learned from it.
I was happy to learn that the paper has been "refurbished" and underwent "General overhaul" to include a better list reference of methodologies and other subjects. It is still one of the most useful references I can find to the overall "subject matter", and a great place to start if you're thinking to yourself "what is this guy smoking?"
Take an hour. Read it.