A book proposal is essentially a mini-business-plan. Its purpose is to convince a publisher to invest thousands of pounds in your book in the expectation of making a profit.
To achieve that, you need to convince the staff at the publishing house — especially those in the sales department — that people will want to buy the book. And the best way to do that is to clearly identify the need for the book.
Write sentences such as ‘The need for this book is…’, ‘The need for that this book seeks to fulfil is…’ or ‘This book is needed because…’
Note that we’re focused here on need rather than mere interest. Lots of people may find your stuff interesting, but when it comes book-selling and -buying, interest is a pretty weak force: need out scores interest markedly. If you can’t identify what the need for the book is, why not abort the project?
You also need to explain how your book is equipped to fulfil the need in question.
The best way to do this is to think of your book as a solution to a problem. Unsolved problems are very motivating things.
For example, these days many people in nursing seek to conduct and publish qualitative research. Yet the institutional context they work in may be more used to quantitative research (typically in the form of randomised controlled tests). Thus the novice qualitative researcher may experience a lack of guidance and of encouragement to hand. This creates a problem: conducting and publishing research is not an easy thing to do, so the researcher is unsure how to proceed. They may be fearful of wasting effort and not doing things well enough. Thus the need is for informed, trustworthy, and supportive guidance at an introductory level.
So you might propose a book to help solve the problem and fulfil the need. You could explain, for example, that your book might save readers large amounts of time by conveniently synthesising information and advice on many topics and from many sources. And you could explain that it provides introductory answers to questions that such researchers typically ask themselves. For example:
- is this worth doing?
- what opportunities might there be?
- what might be the first steps?
- what pitfalls are there?
- where can I go for further (say, less introductory) guidance?
So think (a) need and (b) problem > solution.
Note that this can only really be done by seeing the book from the point of view of readers, rather than that of the author. Learning to genuinely get out of an author-centric way of seeing a book is one of the major steps in successful proposal writing.