As yet, there is no book “Programming in LaTeX3”, and even if such a thing existed there would be lots of gaps. So there is no comprehensive support.
However, there are some ‘resources’:
The documents are still subject to development; some of the broader design issues are discussed on the LaTeX mailing list latex-l — you may subscribe to that list by sending a message by sending a message
‘subscribe latex-l <your name>’to listserv@urz.Uni-Heidelberg.de
This answer last edited: 2012-11-29