creative writing - How do I improve my knowledge of English well enough to write in it?

I have a long-term goal to write a non-fiction book: a critical book about the current state of humanity's moral methodologies and how humanity is better off dead. Much like Friedrich Nietzsche but with a persuasive writing style.

To this end, how do I learn new words? What kinds of books should I read to feed more knowledge into my brain to improve my knowledge of English? Should I be reading books about English (like style guides)? Should I read examples of the genre I want to write? General English-language writing like newspapers, magazines, or blogs? Something else?

Note: I was referred from the English forum to here. English is my second language.