How to Write a Book Proposal
To write a book proposal, provide an overview that describes the book's themes, content, and organization. Start by creating a Table of Contents, a formal outline, and a brief description of specific chapters. Next, include sample chapters to highlight your writing style and overall aesthetic. Then, add an "About the Author" section with basic bio information. Finally, write a brief cover letter tailored personally to each person or agency that you submit the proposal to.
Pick an appropriate project. Generally, books that are published on proposal only are nonfiction books, textbooks, and children's books. Usually, poetry collections, novels, and collections of stories are not submitted in proposal form, because those forms are more about the aesthetics and execution than the topic. Presses regularly look for projects to invest in that address topics or issues that they're interested in. Pick a subject area in which you have credibility. You want to write about something that you're either an expert on, or have become an expert about. If you want to write about the Civil War, but you haven't read the necessary literature, or haven't done any formal coursework in American History, your credibility will suffer. Why should they trust that your project will be successful, interesting, and marketable? Unless you've published lots of work, the strength of your proposal will be built on essentially three things: The strength of the topic and angle The marketability of the book and the interest of the press in the subject Your credibility as a writer Find a wide angle on your topic. Successful books make specific and narrow topics universal. The average reader might not necessarily be interested in knowing a lot about salt, but the bestselling book "Salt: A World History" by Mark Kurlansky managed to find connections between salt and the construction of the modern world. It was a successful book because it made something mundane and specific applicable to all sorts of issues and places. Alternatively, look for a really specific angle and only research smaller presses that cater to that publishing niche. If you desperately want to write about The Rolling Stones drug habits in the summer of 1966, it might be a hard sell to Norton. Drag City, Da Capo, or 33 1/3, however... Pick something you'll be able to spend months or years working on. Are you still going to be interested in researching what the second-in-command Union lieutenant at Appomattox ate for breakfast on the third day of the battle, six months from now? If not, the project might need to change slightly. You need to propose a writing project that you're going to be able to maintain a high level of enthusiasm for throughout the entire writing process. Plan to cover as many of the expenses as possible yourself. Say you want to write a non-fictional account of building a life-size recreation of Noah's ark, or your attempt to start an organic farm from scratch. If you're not widely published, it's unlikely a press is going to help you out financially with the considerable budget necessary for such a project. Are you going to foot the bill yourself? Maybe instead of doing so much personal-journeying legwork yourself, it would be better to find a third party to observe and study. Instead of starting an organic farm from scratch yourself, could your project work by observing a farm in progress? Consider alternatives. Research appropriate venues for your project. Start by looking at publishing houses and academic presses that have issued books on similar topics. Alternatively, you might check out presses that you particularly like, that you're very familiar with, and that you think might be interested by your aesthetics and your project, even if it's not something they've necessarily published before. Check whether or not they accept unsolicited proposals from writers. If you can't figure out from their online presence, find a contact and write a professional exploratory email asking about their proposal policy. In this, you might include a brief author's note and a short (one or two sentence) summary of the project to let the contact know which editor to forward your question to. Start your proposal with a cover letter. This should be brief (250-300 words) and personally tailored to each press, agent, or editor to which you're submitting a proposal. In your cover letter you want to introduce your project and yourself in a few sentences, guiding the reader into your proposal. Let them know what they're about to read. Make sure your cover letter includes: Your contact information Your basic credentials, although not a detailed biography An introduction to your project The working title of the project Some discussion of why you're proposing the project to this specific press Provide an overview of the entire book. Depending on your project, the meat of the proposal will be a basic walkthrough of the themes, content, and organization of the potential book. This might include a Table of Contents, a formal outline, and a brief description of specific chapters that you'd like to develop. The overview should also include sections that address the intended audience and some discussion of why the press would benefit from investing in your project. Describe the market for your book. Who is it written for, and why will they be interested? List your competition and explain how your work differs from theirs. This is essentially your unique selling feature. Include any sample chapters. In the overview, you'll want to include chapter-by-chapter descriptions (as you now see the project) for the entire book, giving the editor some sense of its breadth and structure. You also want to give the editor some sense of your aesthetics and writing style, so it's a good idea to include any finished chapters you've included, especially toward the beginning of the project. Be prepared for criticism. From something as small as the title to as big as the nature of the project itself, editors will have opinions that they're going to be free to share with you if they're planning on thinking of the project. Prepare yourself to confront dissenting opinions and ideas regarding your writing. Include an "About the Author" section. Detail some relevant information about yourself and your credentials. List a basic bio, getting in depth in regard specifically to your expertise in the subject matter. Any formal degrees you've received, previous publications, or research grants you might've received would be applicable and important to include. Include a SASE for their convenience in replying. If the press is interested in publishing your work, chances are they'll get in touch via phone or email. If they choose to pass, it's likely they'll not get in touch at all in a personal way, unless you make some extra effort. Since it's good to know that you can stop waiting to hear back from them, it's good form to include a self-addressed and stamped envelope in your proposal packet, so they can include a short note letting you know they've decided to pass. Personalize your form proposal and cover letter. The more individualized and personal your proposal, the more it demonstrates a real familiarity with the ventures of the press and the kinds of work they publish, the more seriously your project proposal will be taken. Some presses provide a list of editorial contacts in different subject areas who field proposals. Address the letter to a specific editor, not "All concerned" or "Section Editor." Taking the extra step to research the press itself will do much to make you stand out in the early stages. Inquire about any supplementary forms from the publisher you're submitting to. Bloomsbury and other major publishing houses have form packets that they require you to fill out to streamline the submissions process. Lots of the information these forms will ask for you'll have already worked out, so submitting to individually presses will become a matter of taking your pre-written proposal and putting it in their form. It's still a good idea to go through the steps of making a "form" proposal first. Consider the benefits of submitting the project to multiple presses simultaneously. It may be tempting to have your project under consideration at multiple places simultaneously, especially if it's in any way time-sensitive. Presses can take several months to respond to the deluge of proposals and projects they have to sift through, though some won't even consider projects that are being simultaneously submitted elsewhere. Find out their policy before submitting. In general, presses don't like to be part of "carpet-bombing" campaigns, in which an author submits the same thing to every single publisher under the sun, hoping something will stick somewhere. Aiming at specific places and doing some thought about why they'll be interested will make your project stand out much more than a shotgun-style approach. Send it, record it, and forget it. Your psychological health will be much more stable if you submit your proposal, record the date in your record of submissions, and promptly put the matter in the back of your mind. The good news will be that much better when it comes. 
Pick an appropriate project. Pick a subject area in which you have credibility. Find a wide angle on your topic. Pick something you'll be able to spend months or years working on. Plan to cover as many of the expenses as possible yourself. Research appropriate venues for your project. Start your proposal with a cover letter. Provide an overview of the entire book. Include any sample chapters. Include an "About the Author" section. Include a SASE for their convenience in replying. Personalize your form proposal and cover letter. Inquire about any supplementary forms from the publisher you're submitting to. Consider the benefits of submitting the project to multiple presses simultaneously. Send it, record it, and forget it. 