The most insightful book I’ve discovered on writing nonfiction is Ayn Rand’s lectures. She covers all steps of the writing process and outlines everything you need to know to write well.
The second best advice is by PG in Writing, Briefly.
Book: https://www.amazon.com/Art-Nonfiction-Guide-Writers-Readers-... PG: http://www.paulgraham.com/writing44.html