The Boron Letters is one of my favourites - you can find it online for free (google) as the Boron family put it online to share their father's teachings. If you're just starting out, This Book Will Teach You To Write Better it's a great start and a summary to look back to from time to time.
Also if you're interested, I have a free productivity email course in which I share how I went from 9 to 34 books a year.
I don't want to self promote, so if this sounds interesting send me a DM and I'll share the link!