This one is still a major moving target; I asked Dan Connolly what the best on-line reference was, and he said:
Here's the path:
http://www.w3.org/
http://www.w3.org/Architecture/
http://www.w3.org/Architecture/#Research
http://www.w3.org/Addressing/
http://www.w3.org/Addressing/#hotlist
http://www.ics.uci.edu/pub/ietf/uri/
http://www.ics.uci.edu/pub/ietf/uri/draft-fielding-uri-syntax-02.txt