Rule number one: if you are publishing a book, you are creating a product. End of story. When you create a product, the intention is to sell the product - most likely for a profit.
Rule number two: if you want the product to sell, it must have customer appeal.
Rule number three: if you want to sell the product, you need to know WHO will buy it - not who needs it but who will buy it. Perhaps everyone truly needs the product, but not everyone will buy it.
More on this topic later.
Denise