Harry Halpin has just published an article XML.com: Reviewing the Architecture of the World Wide Web on O'Reilly's XML.com site.
It is strange that the formal architecture document, AWWW (Architecture of the World Wide Web) released in December 2004, authored by the TAG (Technical Advisory Group of the W3C) has come 10 years after the event, so to speak. This is becuase the web arose from a series of specific specifications (i.e. definition of a url, http, and html) rather than an overall architecture. The best practice has grown, to some extent organically, around the core.
This article puts the AWWW in context and summarises its key points, and is thus very useful.