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 « introduction to LaTeX3 ideas » in the expl3
documentation gives a broad-brush overview of the concepts.
Joseph Wright has written a short series of
[[https://www.texdev.net/index.php?s=programming+latex3|blog posts]], which may help.
There is also a complete command reference in the
:ctanpkg:`interface3 <l3kernel>` document.
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//>''
Source: LaTeX3 programming