...if it's nonfiction. Y'all know this, yes?
I met someone last night who didn't know this, who is writing a how-to book on educating kids. He thought you had to write the entire draft.
That's really only for fiction and memoir. Forms where the dramatic arc is vital.
How-to books, service books....you do a book proposal. About 50 pages. I sold Pen on Fire on the basis of 50 pages. Of course I had many more because I had been working on the project for years. But I only submitted the best chapters with the proposal.
New writers--authors-to-be--think it's more impressive to an agent and editor if they've writtent he entire book. But no. An agent may want to shape the project differently. And your editor will need something to do.
One of the best books on how to write a book proposal is called just that and it's by Michael Larson. It's been in print for years. I used it for Pen on Fire.
There are exceptions to every rule, but unless you are an exception, follow this rule.