Three things should be in your covering letter:
- You
- Your book
- Reasons to publish it.
You
When writing about yourself, aim for a paragraph summarising relevant points about what qualifies you to write this book. If you want to say lots more about yourself constrain the extra words to a separate C.V. to make sure your covering letter isn't so long that it gets confused with the book itself.
The publisher needs to know if you have had any publishing successes before, if you have any relevant qualifications for writing your book, and if you plan to write any more books on the same subject.
Your book
Describe the book in one short paragraph. There is plenty of space to explain its intricacies in more detail in the synopsis. Here is where you write a short 'teaser' that will intrigue the editor sufficiently to make them want to know more. Imagine you're writing the publisher's blurb that they will use to describe your book on the back cover, in their catalogue and on their website. There's no need to give away the full plot – leave it open for the editor to wonder how it resolves.
Then add a short line to describe how you envisage the book in its printed form. Describe the size, binding, number of pages and price that you think it should be. If those details are similar to a bestselling book or to another book by the publisher you're writing to then make that comparison. It makes it easier for an editor to visualise the kind of book you're proposing.
Reasons to publish it
List any significant reasons why the publisher's risk will be small in taking on your book. Perhaps you have contacts in the media who have promised to help you publicise the book. Or you've done some research into the likely level of demand for your book and can demonstrate that the market is crying out for it. Maybe you're involved in a society whose membership is sufficiently large to create enough guaranteed sales to justify a full print run. Or you're a lecturer on a subject and are sure the students at your college and others will all buy this book. Or you have the ability to sell the book through your own business.
Taking this principle further, imagine you planned to purchase a substantial number of copies of your book at the author's discount (usually between 35% and 50%) for selling privately to friends, family and colleagues or to sell in your own shop. You mention to the publisher that you think you can sell 500 copies of the book, should it get into print, and that you would make a firm order to buy those copies, non returnable, so that they can be delivered direct from the printer. If your book is up against similar, equally well-written manuscripts for consideration, this kind of offer could clinch the deal for you. It won't help you if the book isn't good enough, or it's wrong for the market, or the publisher doesn't specialise in that kind of book, but it can make a difference in the right circumstances.
What kind of a difference does a guaranteed sale of 500 copies make to a publisher? Actually quite a lot. It depends on the size and price of the book, but in most cases it's enough to cover as much as half of the printing costs and therefore reduces the chances that the book will lose money.