A vast number of XML resources are on the Web. In this appendix, I can give only a few that are either generally useful or that link to other useful sites.
The place for definitive information on XML can be intimidating and overwhelming. If you are a newcomer to XML, visit this place from time to time, but not to find introductory tutorials.
From the top of the URL, you can link to Technical Reports, which is where you can access up-to-date drafts of a whole raft of XML technologies. Also linkable from the home page is a page describing W3C “Activities,” which gives a perspective on the thinking behind much of the current and likely future W3C activity.