I see lots of pitches from writers who have written complete non-fiction manuscripts for all variety of books.
Sometimes that works out fine, and I’ve certainly repped and sold complete manuscripts. But for the most part, considering that the great majority of proposed books are not placed with a publisher, writing your book before you have a publishing contract can be a misuse of your time and energy. Most agents certainly think so.
I always encourage prospective non-fiction authors to focus their attention on first writing a strong book proposal and sample chapter, a process that in itself than can be a long haul, but very useful in terms of clarifying your purpose and answering the kinds of questions a publisher will have for you.
Don’t kid yourself, a great idea or manuscript goes only so far, and publishers will look as closely at the market, the competition, your platform, and your credibility as they do at your writing sample. Not only that, even if your idea is just right for a particular house, you may find that they’ve already signed something similar from someone else who got in the door first with a proposal while you toiled away on your manuscript. Or you may find that an editor loves the concept but wants to tackle it in an entirely different manner, and so you end up re-writing anyways.
If you write for love — and maybe even if you’ve pitched your book once and it didn’t fly, but you’re determined to see it published one way or another (on demand for instance) — feel free to work on it, but don’t assume that a finished manuscript is any more marketable than a great proposal.
The exception proves the rule disclaimer: there are a number of books that have defied this wisdom and were ultimately self-published before making it to the big time, The Celestine Prophecy, and Mutant Message Down Under among them.