steps toward the glory of REST
A model (developed by Leonard Richardson) that breaks down the principal elements of a REST approach into three steps. These introduce resources, http verbs, and hypermedia controls.
... read the whole story at martinfowler.com.
Recently I've been reading drafts of Rest In Practice: a book that a couple of my colleagues have been working on. Their aim is to explain how to use Restful web services to handle many of the integration problems that enterprises face. At the heart of the book is the notion that the web is an existence proof of a massively scalable distributed system that works really well, and we can take ideas from that to build integrated systems more easily.