While some people look through the lists of books that have already been published and complain that everything has already been done before, you and I know that isn't true.
Let's look at the subject of growing tomatoes. There are already some great, all-purpose guides published on growing tomatoes. So what did some ingenious writers do? They wrote books on growing tomatoes in containers, how to grow world record tomatoes, growing heirloom tomatoes, and growing organic tomatoes. And that's just the tip of the iceberg as far as niches for growing tomato plants.
The trick to being able to write a book about a popular subject is to find a niche for it.
Target Specific Groups
A common way to find a niche for your book is to target it towards a specific group of people. Examples of this would be single moms or dads, women or men, city slickers or rednecks, people with straight hair, curly hair, old timers, children, people with a limited income, and people who love luxury. The list goes on and on. So, if you want to write an original book on growing tomatoes, why not write the redneck's guide to growing tomatoes and talk about the finer points of growing tomatoes in old tires.
From beginner to bloody brilliant expert know-it-all and everything in between, these are the basic characterization of skills. You can create books for people under each skill set, targeting tomato growing towards people who have a black thumb or targeting a tomato book towards people who think they already know it all, but are willing to see if maybe, just maybe, there's a little more to learn.
You can target specific states when writing your book. For example, starting a childcare business is different from state to state because of state regulations. If this is something you know a lot about, you can write a book on childcare that is specifically geared toward your state.
What it all comes down to is making your book unique. With a touch of determination and a bit of creativity, you can write about any popular subject and walk away with a one of a kind book.