This is probably way too basic, but at least it’s technical: https://web.archive.org/web/20170916193014/http://www.adarshr.com/fun-with-xmpp-and-google-talk and the second part, https://web.archive.org/web/20171005104211/http://www.adarshr.com:80/fun-with-xmpp-and-google-talk-part-2
It explains what stanzas are, what types are available and stuff.