I love organizing really big messes. Not closets or garages. The messes I like to organize are massive amounts of content. A sentence here, a paragraph there, and a transcript elsewhere, and they all need to be synthesized into a coherent, compelling document.
Pick one ideal person, maybe a client or someone you know, or a hypothetical ideal reader, and think about what you want to tell her. When you know how old she is, how much she already knows on the topic, and what you want her to take away from your book and do in response to your writing, you will be in a much better position to collect your thoughts.