Ginny,
If you don't have a lot of experience with researching nonfiction, ANATOMY OF NONFICTION is very helpful.
In terms of the craft of writing nonfiction, I think FINDING THE HEART OF NONFICTION (Georgia Heard) is really the best. And one of the best things you can do is look at nonfiction published within the last three years and really dig into its structure at the book level, sentence level, etc.
Best of luck,
Kirsten