Many times people come to me and tell me they have this great idea for a book. They are certain it's going to be a New York Times Bestseller. They ask me, what do they need to do first to accomplish this writing goal. Most people answer that question with, "write the book." Or "write a good book." But before you ever write the first sentence you need to ask yourself the following three things:
1) Who is going to read your book? Books are products so you need to think like a salesman when you think about your books. It's easy to do a little research. Who is your target audience? Self-help readers? Mystery readers? Romance readers? There are plenty of associations that can tell you exactly how many readers there are. Why is this important? You should have an idea of how wide your audience is. Keep in mind that that audience has a budget and will divide their book dollars by the writers they already know and love and new writers to try. Can you figure out who gets the bigger portion of that budget?
2) Where will it land on the shelf? If you want to write a hybrid science fiction/horror/romance, on what shelf, exactly, would a book seller place this book? Is there a published writer writing a comparable work? If you can't place it on a shelf, you risk not being able to get a publisher to even look at it. No matter how well written. Books are products and publishers are manufacturers who think about the audience above all. A trip to a local bookstore can help you identify that shelf.
3) Who will publish your book? Again a little research. Look at books like your book and see who the publisher is. Go to the publisher's website and see what types of books they are looking for and check out the submission guidelines. That way you won't write a 500 page book only to discover your targeted publisher is looking for 380 words. Or if you write 50,000 words but books published by your targeted publisher need to be at least 70,000 words. This information is usually available in the submission guidelines.
If you choose to self-publish, you need to look into how to do that, where to do that, and what to expect before, during, and after you self-publish your work.
If you can answer all three of these questions you are ready to write your book. You've also begun to think about who you're writing your book for and how it will get published. It's hard to think of your art in terms of a product, but ultimately that is what it is and knowing the answers to these three questions will help you produce a better book before you write the first word.