Unfortunately, that's what language docs are for. Like I said, you can get some theory. If you're learning functional programming, you can find books on lambda

Unfortunately, that's what language docs are for. Like I said, you can get some theory. If you're learning functional programming, you can find books on lambda calculus and category theory and more. But ultimately you have to learn how to build good software, and there are scant few books about that out there. In the functional world, I know of two and they're both in Scheme. So the best approach tends to be feet-first into the fire using the language's docs, and hopefully the creator(s) has documents explaining theory and practice.