In the days when I imagined I could write fiction I tried to plot out one story, and killed it dead in my imagination. I found I could only write fiction if I knew where it started, and that was all. I had to be discovering what was happening in order to keep the whole thing interesting enough for me to want to continue.
Non-fiction, on the other hand, I need a plan. I have the chapters worked out, I have a notion of what will be covered in each chapter; though I will almost invariably change this during the course of writing. My book on Chris Priest, for instance, I had a very detailed plan done in advance, wrote the first chapter, made a start on the second chapter, and ground to a halt. It wasn't working. I had to scrap what I had written, scrap the whole plan, and concoct an entire new plan on the go as I was writing the book. The writing, of course, is the easy part; I have very detailed notes (done in Evernote), and the notes grow organically as I am researching, so when it comes to putting words on the page a lot of what appears is simply copy and paste from my notes.