Put a large Post It note in the middle of the board; write your topic or tentative book title on it. Write related topics on smaller sticky notes.Use one color for this part of the exercise. Just stick them all over the board. Don’t worry about where you place them , or about organizing them, unless you are certain they are related topics; then you can group them together. This is the “brain dump” phase. Put everything you can possibly think of onto the board. Free associate. Fill the board to capacity. When you think you are done, think again.
If you haven't tried mind mapping for your book, read this article. You'll be amazed at how effective it is.