How can I learn advanced LaTeX and become a full LaTeX programmer?

Here's a (possibly opinionated! :-) ) list of books/resources that might be helpful:

• Check out the third part of our online \(\mathrm\LaTeX\) course
• The LaTeX wikibook covers quite a bit of advanced stuff, including lower-level topics like lengths, boxes, macros, creating packages, etc.
• Stefan Kottwitz's LaTeX Cookbook is nicely structured, with a lot of good examples.
• The classic LaTeX Companion is still a classic, in our opinion.
• Nicola Talbot's new book LaTeX for Administrative Work is a lot of fun—automating many tasks with LaTeX.
• No LaTeX book list is complete without The TeX Book and TeX by Topic, especially if you're interested in the actual working details of TeX and LaTeX
• Take a look at this TeX.SX question for more inspiration!

Also, hanging around on TeX.SX to browse and answer questions would also help gain quite a bit of experience. You can also follow Overleaf's twitter feed about our Gallery updates for interesting examples.