That depends a lot on your current level and field. I myself am a junior frontend developer. I started from scratch through freeCodeCamp.org and I passionately

That depends a lot on your current level and field. I myself am a junior frontend developer. I started from scratch through freeCodeCamp.org and I passionately recommend it to anyone to starting to learn to code. If you passed that stage, I recommend the book The Pragmatic Programmer as a very good book on how to become a good software developer. The book Clean Code is a good starting point too. Other than it depends on your circumstances. A good heuristic (particularly in frontend world) is to study the language, not frameworks.