Frequently Asked Question List for TeX
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’:
expl3documentation gives a broad-brush overview of the concepts.
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_>
FAQ ID: Q-latex3-prog