In the fiction-writing community, there are two terms thrown around quite a bit: plotters and pantsers. Plotters are writers who know the whole plot of the story before they start writing. Pantsers are those who write “by the seat of their pants” and prefer to start with a situation, then discover the plot as they write.
These two categories also apply to other kinds of writing, including non-fiction. Planning and outlining are important, but they won’t take you through the whole journey. The real discovery process of writing comes in the actual writing, when you are fleshing out ideas in sentences, paragraphs, and chapters.
Henry Miller said, “Writing, like life itself, is a voyage of discovery.” A good outline, especially in non-fiction, can be vital but don’t get stuck there. Set your course, then start writing. The discovery of new ideas will come as you write, but rarely before.