My experience writing a 1,200 page technical book was similar to xrd's. It was worth it in the end but I'm not doing it again either. Jerry Seinfeld's "Don't break the chain" is good. I did take days off though (on average, 1 a week). I found this book very helpful even though the genre of book was radically different: https://www.amazon.com/2k-10k-Writing-Faster-Better-ebook/dp...
Wrote the latest applied crypto book over two years and a half. I love writing, and I write all the time, but that was extremely tough on me. I would not do it again.