"Tips for Writing a Technical Book" https://news.ycombinator.com/item?id=20070558
"How to write a programming book" https://news.ycombinator.com/item?id=23281568
"Learning technical writing using the engineering method" https://news.ycombinator.com/item?id=22283919
And a GitHub list: https://github.com/jenniferlynparsons/awesome-writing