The normal course of business in the non-fiction world, whether published or unpublished, credentialed or non-credentialed, is proposal plus 1-3 sample chapters.
Lets not forget that this thread is based on work coming to an editor from an unpublished author. I agree that what you describe above is the normal course of business for all, and that it does give an editor a clue as to whether the writer has some ability to write. But it's not an end all. The proposal and sample chapters should also give the editor an indication as to whether or not the writer has material worthy of publication. But it's not an end all which can be applied to all unpublished writers. For these and other reasons I believe that most editors--if intrigued by a proposal and sample chapters--will in most cases want to read a complete manuscript when dealing with an unpublished writer. Frankly, I think it's possible to write an excellent proposal and 1-3 sample chapters and not be able to write an entire book.
No editor is going to read through an entire non-fiction book just to see if you can write. That would be the exception and not the rule and, frankly, I've never heard of that before.
Right, that's why they first want a proposal and sample chapters. They don't have time to read entire manuscripts from all who submit. It's something I've heard of quite a lot.
If your proposal does not indicate via chapter summaries and outline that you have enough for a book, they (or your agent) will let you know. Many agents will say "I don't see this as a whole book."
It may indicate that you have enough for a book, but being unpublished how is an editor to know you can do it? If you are a well credentialed person within the field you are writing about they may take a chance.
The tricky part about writing a proposal is convincing both agents and publishers of what they cannot see. That's why a proposal is so much more important than an entire manuscript. If you can't convince me with a proposal, you won't be able to write the book.
For openers the proposal is more important than the manuscript. However, in the end it's all about the complete manuscript.
I hated the proposal so much but as I went through it agents and editors said this to me over and over again...if you can bang out a great proposal, the book will be cake.
I guess this is something we'll just have to agree to disagree on. I don't think a great proposal guarantees a great complete book.
And it was true.