Effective Writing for Engineers, Managers, Scientists, by H.J. Tichy.
It was fantastic for me because it describes writing as an iterative process with subsequent rounds of structuring and refinement, and that helped me to overcome blocks. I learned also a lot on how to make my writing more concise and to the point.
For a scientist and programmer, it is really essential that one does not simply assumes that one already knows how to do it! For me, it is way more difficult than it looks - but also way more satisfying than I thought, when it is finished.
I agree that this is not for everyone, because it needs to be learned, is hard to learn, and is a very different set of skills to coding. And there is clearly a field of work for specialists.
One lesson learned that I took away from this is that writing is a separate task, and it is difficult to do it at the same time as producing code.
But back to the main topic. As a documentation writer under time constraints, what could I do to find out what the not-yet-present reader of the documentation might need to know?