Since the plot of a nonfictional history is largely fixed by actual events, my own method is character-driven, with information collected into profile sheets for each of the dramatis personae. (I use the term “sheets” loosely, since the one for the horse Epinard is already 27 pages long). Having learned from grim experience about the difficulty of relocating references (especially the ones I was “sure” I’d remember where to find), the sheets also contain all the necessary information for the endnotes.
I won’t use all the information I collect, of course; some will prove irrelevant or redundant. Still, I’d rather have too much and have to pare it down, for this is the foundation of my writing, and I’ll probably spend at least four hours researching for every one I spend on actually writing the book (and that’s likely an underestimate of the ratio). Fun? It is and it isn’t. It’s a treasure hunt for the interesting and the unexpected; it’s also a slog. My hope is always that the quality of what I do at this stage will be reflected in the quality of the end product: a good book.