What's the key to writing a book that sells? That's simple.
You must write a book your audience will want to read.
Quite often I meet writers who don't concern themselves with their readers as much as they do with the "message" they hope to impart to these readers via their book.
There are a couple of HUGE problems with this way of thinking.
First, if you write a book mainly because you have a strong message for your readers, your book will tend to become WAY too preachy.
When this happens, you won't find a publisher for your book because good publishers realize that readers do not want preachy books.
Second, if you choose to self-publish your book (which you'll have to do since this is the only way you'll be able to get your message out to the world), then you'll still end up with a preachy book no one wants to read.
But now, you'll have a basement or garage full of these books.
A good coach can help you avoid these problems by showing you how to figure out just what your audience wants to read.
You can still weave "your message" into your manuscript.
But you will learn to do it in such a way that your readers will think your only reason for writing your book was to provide them with the help they needed.
When you write a book like that, you'll have a book publishers will be willing to publish and readers will be willing to buy! So, don't just write a book.
Learn to write a book that will sell!