Can I ask a question? As agents, do you prefer the book to be outlined, fully written, or partially written for NF queries? A friend and I are starting some work on a draft, and while I’m familiar with queries for fiction, this whole NF thing is brand new to me. Thanks!
First, if non-fiction is new to you, time to dig in for some research.
Any book on how to write a non-fiction book proposal will be useful. I prefer the one Elizabeth Lyons wrote.
Absolutely essential however is Susan Rabiner's book. Stop what you're doing and read it first.
As to your question; it depends.
Are you writing a memoir?
Your book needs to be finished, just like a novel.
Are you writing narrative non-fiction (history/biography etc)
You need an overview about the subject that answers the questions of what you're writing (what question you're answering, what problem you're solving), and why it's important NOW (significance.)
Example: narrative history COLUMBINE by Dave Cullen. The significant question here is what really happened, and why. It's important because only when we know the why can we start to address the problem.
You need a list of chapters and projected contents of each chapter.
And you need a section on your platform and qualifications to write this book.
Are you writing prescriptive non-fiction? (how to, instructions, guide to better living, advice, etc.)
You need an overview (as above). You need a chapter list (as above). You need platform (as above)
and you need success stories.
Example: The QueryShark Guide to Effective queries proposes to help writers create effective query letters. Platform is the number of readers who consult QueryShark.blogspot.com. Success stories are the folks who not only got to yes, but those who credit QueryShark with securing agency representation. An actual list of those folks would be in the query.
So, the answer, like always, is what you send me depends on what you're writing.
And this applies across the board; it's industry standard.