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 <a href=“mailto:listserv@urz.Uni-Heidelberg.de”