Ethan Marcotte defined responsive web design in his seminal book, Responsive Web Design. It’s still the best place to start to understand the core concepts.
Aaron Gustafson demonstrated that responsive web design was just the tip of the iceberg in Adaptive Web Design (essentially, pushing “content first” through progressive enhancement).
Now Tim Kadlec in his book Implementing Responsive Web Design gives the most up to date practical guidelines for designing and developing modern future friendly websites. Contributions from Aaron Gustafson, Luke W, Brad Frost and Guy Podjarny amongst others.