Which books, courses, projects have improved your programming skills immensely?

Have you had any experience as a mediocre programmer where you coded mostly hit and miss style and then you did some project, read some book(s) or took some courses and you started to program like a scientific process and a mental model was formed and eventually became a good then a better programmer?

You started understanding programming languages much easier and better and were able to structure solutions in more elegantly and efficiently in code.