Lately, most of my active project work has involved mapping content to parts, chapters, and sections of books. In other words, figuring out what a particular book should contain and in what order.
People get stuck here. It's overwhelming. You need a solid process or you're going to get lost in the weeds. Navigating authors through this across so many books and book proposals over the years has led me to a workable approach, so I figured I'd share the 101 version here.
Rather than create an arbitrary top-down outline, start from the bottom. Begin by gathering all the material you've accumulated—notes, research, transcripts, whatever—in one place. Go ahead and just dump it all into a single document. Don't be shy. If it's remotely relevant to your idea, shovel it in there and don't let yourself get sucked into organizing it in any way.
Next, comb through this information dump methodically from the top, cutting and pasting each discrete nugget (case study, personal anecdote, whatever) into the appropriate "bucket." What's a bucket for our purposes? A subject, a direction, a line of thought. It might be chronological: "The 80s." Or simply descriptive: "Investing in index funds."
In the final manuscript, a single bucket might amount to a paragraph, a section, a chapter, or even multiple chapters. It's too early to know until you've assembled everything that belongs in one, so it's helpful not to label your buckets in any of those ways. Stick to "bucket." Once a bucket contains absolutely everything you might want to say about that subject (or sub-subject, etc.), you'll have a much better idea of its scope.
How do you create your buckets in the first place? Simple. Start with no buckets. If you have nothing to say, you're off the hook. Find another hobby. As soon you do have something—an idea, technique, fact, or finding—consider your various taxonomic options and choose one. Don't obsess over it. You can always make new buckets and get rid of old ones as you go. The important thing is to start sorting because the sorting process itself will guide you. Each time you isolate a piece of material that belongs in your book, either add it to an existing bucket or, if it doesn't fit anywhere, start a new one. Keep divvying up the raw material this way, snippet to bucket, until each and every piece you've accumulated has a home.
Once your array of buckets is sufficient to contain all of your raw material, merge and subdivide your buckets as needed. One overstuffed bucket might break apart into five smaller ones, potential chapters. Another might amount to a few paragraphs at best. Pour that bucket into a larger one. Just do your best to even things out at this stage.
In some cases, you can't really tell how much a bucket really contains until you start writing a draft in earnest. No worries—just start writing it out. Did you get to five hundred words? Or five thousand? Reframe it as a part, chapter, or subhead accordingly.
Once you're confident that every piece of material you've explicitly gathered for your book has a clear and obvious home in at least one bucket, turn to more traditional outlining. For this, I find it helpful to look at the table of contents of similar books. We all have blind spots, areas of knowledge so obvious (to us) we don't even realize they bear mentioning. Reviewing comparable TOCs helps spot them.
Say you're a Special Forces commando writing a manual of self-defense for civilians. One of your buckets might be "Travel." There, you'd gather together advice ranging from protecting yourself on the subway to staying safe in poorly lit parking lots. (Of course, your brain might dictate two buckets here: mass transit and cars. That's fine. Let the way you think about your subject lead the way.) Another bucket might be labeled "MacGyver." There, you'd discuss the repurposing of house keys as weapons and the use of belts as improvised tourniquets.
Once you've sorted your material into buckets, it would be easy to assume you're done outlining. Not yet. A glance at the TOC of another self-defense book might cue you to consider adding a "legal" bucket. Seeing a chapter on the law in another self-defense book might spark a whole series of related ideas you hadn't considered discussing, from the effect a black belt can have on a manslaughter charge in some jurisdictions to the politics and history of the Second Amendment. Identifying a missing bucket doesn't mean you have to include it, but far better to spend time evaluating the option than to accidentally build an entire draft (or even publish a book!) with a gaping hole.
In short, to build your book, think bottom-up, then top-down. The beauty is that you never have to consider your book in the abstract. You can start with any bit of material you've got in mind: a quote, a statistic, a method. All you have to do is lump it together and mechanically sort through it. Through this sorting process, a pattern will emerge, and—in a very limited sense—your book will write itself.