I can say I know how to create documents in LaTeX. However, I want to learn more. I would like to know how to learn to create my own packages. It may be a bit presumptuous for a beginner, but at least I would like to learn to do some of my own things! For instance, make beautiful designs for chapters, write codes (macros), etc. What language is used?
I would be grateful if you can recommend some good book.