is a W3C-Recommendation. The Primer has today's date. It's been under development for a while (somewhere near three years, I think).

Important new things: value-adding intermediaries; a REST pattern; and more. This is actually a really nice concise description of REST:

One of the most central concepts of the World Wide Web is that of a URI as a resource identifier... For example, a very important - indeed predominant - use of the World Wide Web is pure information retrieval, where the representation of an available resource, identified by a URI, is fetched using a HTTP GET request without affecting the resource in any way. (This is called a safe and idempotent method in HTTP terminology.) The key point is that the publisher of a resource makes available its URI, which consumers may "GET".