Resources for learning how 'advanced level' stuff in LaTeX

I oftentimes only see dense resources (books/lengthy tutorials) about how to use LaTeX. They generally spend most of the time teaching how to make tables, use math mode, cite papers, and other basic things you would want to learn when first using LaTeX.

I would like to hear about more dense resources that teach more advanced topics, like for example
making conditional formatting functions for tables or making a documentclass.

Is there any single resources that can take an intermediate level LaTeX user to advanced level?