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A friend of mine has spent many years developing several treatment methods for certain psychological disorders. She believes individuals with these disorders would benefit if she could write a nontechnical book that consolidates many of the principles she teaches to her patients. She is striving for a book that sort of summarize what tools are out there and how best to use them for people with those disorders. In other words, she wants to write a layman's book but one that balances being engaging in terms of writing but that still conveys the core of her theories.
Are there any go-to resources on how to accomplish this? Specifically, a book that explains the subtleties of technical versus non-technical writing and how to blend the two together.
Think like your patients. Look at their problem from their perspective. You can already do that, or otherwise your therapies wouldn't work.
Structure the content. That is, create a narrative similar to the underlying plot of a novel: develop a plan of what you want to say in what order. Looking at the problem from the perspective of your patients will guide you in that this gives you a starting point (problem but no idea what the problem is and how to solve it) and a goal (no problem) and how to get there (what you do in your therapy). So you already know the "plot", too.
Write the book. Use the same language that you use in your therapy. If your patients understand you then, they will understand you in writing. Again, you already have what you need.
Add interesting case examples or whatever to help your readers better understand the theory and how to apply the methods. You got that.
That's basically it, and you have all it takes. Everything else is your ingenuity and creativity, and that must come from you.
If you cannot form pleasant sentences, or have no idea how to make your book fun to read, or don't want to invest the time to learn the skill of writing, then hire a ghost writer who brings that kind of creativity and expertise.