RESTful Web Services by Leonard Richardson & Sam Ruby.
REST is nothing new, it’s been around for at least five years, but it’s getting a lot of play right now, maybe because some of the big players like Google are deprecating their SOAP services.
For a readable explanation of REST as a web services architecture, see this .ppt presentation at xfront.com by Roger L. Costello and Timothy D. Kehoe: http://www.xfront.com/REST-full.ppt.