The "blurb" is the back cover material for your book --- the selling points that will get people to buy the book. If you write the blurb before you write an outline, you're guaranteed not to wander off the track as you write your book.
Here are some reasons to write your blurb first:
A) it keeps you focused on the theme of your book;
B) it makes writing the outline easier;
C) it makes selling your proposal easier;
D) it will assure your agent and editor that you know what you're doing, and they'll feel comfortable working with you and handing over the advance;
E) when you've sold the book, and the time comes to write it, you'll have an easier time because you can keep the blurb at the forefront of your mind.
Your blurb helps your agent and editor to get a contract for you. Your blurb is the "sales story" for your book. If your agent becomes enthusiastic about your book, she'll become enthusiastic on the basis of your blurb. She'll use the blurb as her sales pitch to other people. For example, when she talks to an editor at a publishing house who may be interested in your book, she'll start with your blurb.
When you've written your book, your publisher will try to sell your book to book distributors, and later to booksellers, all on the basis of the blurb that you started out with. So the time that you spend working on the blurb is not wasted, it's the most important part of your book. Without a good blurb, your book will not come into existence.
WRITE YOUR BLURB IN EASY STEPS
Before you start writing your blurb, ask yourself: who will be reading this book? This question is important, because it helps you to picture the reader as you write. Once you have an image of your ideal reader in your mind, you'll find it's much easier to work on your book. Working out who your readers will be also gives you a head start in writing the marketing section of your book proposal.
Let's stay with the book on natural healthcare for pets. Who would be interested in this book? Make a list. Your list could start with: pet owners who use natural healthcare, companies that manufacture natural petcare products, and veterinary surgeons.
Then go on and create your blurb in the following easy steps.
Step 1: Make a list of the benefits to the reader
Your reader will buy the book because of the benefits the book gives her. Features are different from benefits. For example, you may be presenting recipes for making pet remedies. The pet remedies is a feature. The benefit of the pet remedies could be that they save the reader trips to the vet and money on expensive commercial products. YOU MUST USE THE BENEFITS IN YOUR BLURB.
First, list all of the features your book will contain. Then make a list of all the benefits. Take down three or four books from your shelves, and study their blurbs. Do they list the benefits? How are the benefits presented? (You'll occasionally find that the author and publisher, not to mention the publisher's sales and marketing departments, were all asleep when the book was in production, and the blurb contains a long list of features. Work out how you'd convert those features into benefits. This is excellent practice for you.)
Step 2: Rank the benefits
Rank the benefits in their order of importance. You may want to get some help here. Read your list of benefits to a friend, and ask how she'd rank them.
Step 3: Write several blurbs, in various lengths
In addition to your list of benefits, your blurb can contain an intriguing fact, or a short anecdote. For example, if you once saved the life of your pet with a natural healthcare remedy, you could tell this story as part of your blurb.
When you've completed your blurb, in around 200 to 300 words, create shorter versions. Create one of 100 words, another of 50 words, and you can even try to pare it down to 25 words.
Step 5: The testimonial
Publishers love cover testimonials, because they know that they sell books. How many times have you bought a book because someone you'd heard of and respected recommended the book to you? If you know anyone famous, or can get in touch with them, now's the time to contact them to ask them whether they'd be willing to read your book and provide a quote for you to use on the cover.