Autopia Car Care sponsors an car detailers forum that has plenty of good information. They have a library of various detailing topics which takes you through the basics from basic car care to pro level detailing:
http://www.autopia-carcare.com/

There is a free downloadable detailing guide located here:
http://autopia-carcare.com/freeguide.html

Finally, there is a user forum with plenty of professional detailers contributing information including product reviews: http://www.autopia.org/. It's a little intimidating at first because of the depth of knowledge, but I haven't found a better detailing site on the web.