Keep in mind that book publishing is first and foremost a business. Those people who are in a position to publish your book are looking for every assurance that they will make a profit on it. Your book proposal needs to convince them that they will.
First the book proposal, then the book
While a novel (especially by a first-time author) or a children's book typically need to be completely written before they are sold to a publisher, many non-fiction books (such as how-to, self-help, the exploration of a non-fiction topic, etc.) do not. If you have a non-fiction book idea within your topic of expertise, you don’t need to write the book in its entirety before finding a literary agent. Instead, you write the book proposal.
On the strength of your book proposal, an agent will judge whether or not you have a salable idea. The book proposal then becomes the document by which the agent sells your idea (and you!) to a book editor. Once a book deal is struck, then you go on to write the book as outlined in the agreed-upon proposal.
Note that, even if you already have a written manuscript about your topic specialty, if you want to get and agent and sell the book to an established publisher, you will still likely need a book proposal. It’s unlikely that an agent or an editor with whom you don’t already have a relationship will take the time to read an entire manuscript without first reading and liking the book proposal.
The goals of a book proposal
Your book proposal should definitively and compellingly convince agents, editors, and other decision-makers in the book acquisition process that you know your subject, you know your audience, that you’ve done your homework — and, most of all, that there's enough of a market for your book to make the publisher's investment in you worthwhile from a profit-and-loss standpoint.
While writing a book proposal might take less time than completing a finished book, it is not necessarily easier. A well-crafted, bulletproof book proposal requires you to think hard about the book you want to write, as well as do some serious research into the specifics of marketplace.
While book proposal format is a fairly specific one, to get started thinking about your book proposal, brainstorm on the answers to these three basic questions:
Why does there need to be a book on this topic? Who are the intended readers? How large is that audience of readers? What are the holes in the marketplace — that is, how are the needs of your intended readers not being served by the books that are currently on the market? How will your book fill those holes? What is your vision of the finished book; when he or she is finished reading the book, what do you want the reader to come away with? Why is the information in your proposed book best presented in book format?
Why are you the absolutely most perfect person to write this particular book? What are your qualifications to write the book you are proposing? What is your media platform? How can you otherwise help the publisher’s publicity department and marketing department to formulate a media strategy and get the word out about the book when it is published?
What are the factors that make the topic of your proposed book timely? (But not flash-in-the-pan timely: keep in mind that a print book usually takes at least eighteen months from proposal to bookstore). What are the growing trends that point to the fact that this book is needed now? And/or, what are the factors that make your book idea timeless (i.e. a perennially-selling backlist book)? What are the other currents in the marketplace and/or media that will support the book? (Again, think of a year and a half from now.)
The stronger your book proposal, the more likely it will sell you and your book to an agent and editor. Your answers to the above questions will be the basis for outlining and writing your formally structured book proposal.