Daniel Pink
Author of seven books on motivation, timing, sales, and regret
Why themWrote speeches and articles before seven books.
Daniel’s advice: About 70,000 words at 500 a day: half a year of drafting, and the rest doubles it.
For me, a book is about 70,000 words. I write 500 words a day, six days a week, so that's roughly half a year of actual drafting. The rest, research, revision, and listening to my wife read every chapter aloud, easily doubles it.