What is the best way to learn to programming in LaTeX3, and where can I get the best information in your opinion. I know that does not sound like a specific question, but I want to deepen on LaTeX and do not know where to start.I've done my research on the web and I have found dozens sites referred to LaTeX3, but which of these are appropriate for someone who starts now?
Currently, we don't have a Programming in LaTeX3 book, and even if we did there would be lots of gaps. So there is no 'complete' solution. However, there are some resources. First, I'd point to the introduction to LaTeX3 ideas which gives a broad-brush overview of the concepts. I have written a short series of blog posts on the topic. There is also the complete command reference, as well as course of the questions here tagged latex3 or expl3.