Just keep going. The advantage of nonfiction is the story is before you once you've done the research. The challenge is in how to present the story. I think of writer's block as -- I don't know what to write, mind is blank. That isn't a problem for me, being a nonfiction writer. My problem is being overwhelmed by so much material. The way I deal with it is to narrow down what I'm working on to easily digestible bites. You don't eat the meal all in one bite, you don't write the entire book all in one sitting. To stay fresh, I set the timer on my phone for 30 minutes and put it across the room, so I have to get up from the computer or book or notes to turn it off.