I have helped a number of creative people with achieving deadlines and project managing what they claim is art that doesnt work that way.
There are lots of techniques to do this, and I am sure you know of most of them.
One thing I did for musicians was start 24 hour composing sessions, which ran globally for a number of years.
Quite a few of the ideas that came out of those sessions have ended up on commercial releases.
Similarly I have helped a couple of young guys getting into film composing project manage their output.
I dont really see a difference between film composing and business in general, an incremental iterative approach is best!
These guys now live in L.A. and work in proper studios on terrible crap hollywood movies, but at least they are getting paid to do what they like to do.
For many people its about removing distraction, not sure if you have tried that, if you were in solitary confinement with only what you need to write a book, would you have achieved more output?
If the answer is no, you have no book to write!
For other people, the issue is perceived lack of quality. It is difficult for many to understand that bad or poor quality work is better than nothing. It doesnt have to be shared or released, it can often be re used and improved.
If you were talking about composing music theres lots of specific techniques involving limiting yourself to form the basis of new ideas, such as 1 string only on the guitar, pick 4 notes at random and only use those, fill out a score sheet with seemingly random tempos or key signatures and then write music to fit, take something you already wrote and re write it by using the same melody or rhythm in a different sequence etc.
Perhaps there are similar techniques to use for writing.
Once you get past that you are into what film makers call storyboarding, what composers call charts. You need a similar thing for any book that will allow you to work on different bits rather than trying to write from start to finish.
Again, I reiterate, you are probably aware of all of these techniques, practicing what you preach is a far more difficult task, ask any preacher