It's common for authors, experienced ones and first-timers alike, to think they need to write a book before presenting their work to literary agents and publishers. While it may hold true if you're writing a novel, things are different from non-fiction books that solve problems – i.e., losing weight the right way, organizing your personal finances, buying the right used car, etc. For books of this type, a well-crafted proposal showing the significant sales potential of the topic is more than sufficient. The best book proposals read like marketing documents that think things through from a sales perspective.
Persuasive book proposals follow an established format to be successful. The first part involves building a case for the book in the first place. You do it by showing there is a need for it in the marketplace and summarize the success of similar titles. The point is to show the book-buying public has an appetite for information about your planned topic. You can divide this section into competitive titles, which means books on the same topic as yours, and comparative titles, which are related but different in some way. In doing this research, you will learn lessons about the viability of your book idea or not.
The next part of your proposal needs to focus on the target audience for your book. To convince a publisher to take on your project, you need to explain who is currently buying a competitive book and why they will be interested in purchasing the one you propose to write. It's helpful to think things through from the prospective reader's perspective: what are the things happening in their lives causing them to need to read the information In your book? If you can find a niche that's related to current titles in print, but that has not been adequately explored, that's when you'll win over agents and publishers.
Once you've made a case for your planned book and described its audiences and why they will be interested, you need to make sure your information is specific. Use statistics and numbers as often as you can because they are concrete and points people can understand. For example, if you're writing a book about parenting during work-at-home hours, it's good to point out that X percentage of parents of children under 12 work from home. Those kinds of hard facts influence the decisionmaker's opinions much more quickly than a general statement from you. You only need to do some research to find your data.