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.