How to Research the Competition Before Your Write Your Book
Before you even begin to write a book, you need to find out what other books 
exist on your topic. If you are pitching to a traditional publisher, this exercise is a required part of the process. But even if you are self-publishing, you should evaluate your competition so that you know how to position your book as different or better than what is already available.
The easiest way to find competing titles is to search www.Amazon.com for keywords on your topic. For example, if you want to write about how to start a consulting business, search the business books category for the following key words:
CONSULTING
COACHING
ADVISOR
START A (insert specialty such as financial planning, marketing, etc.) BUSINESS
If the market is saturated with books on your topic, consider narrowing your focus. For example, if you run a pet-sitting business, you could write a book called, How to Start a Profitable Pet Services Business and include chapters on pet sitting, obedience training, grooming, and dog-walking. If the market is over-saturated with books in this genre, you could change your focus to How to Market a Pet Services Business or Real-World Advice from Pet Business Owners.
Be sure to poll your potential audience. Ask business contacts and customers if they would be interested in reading a book like yours. Find out what kind of information they want to learn about. Study your market carefully before you proceed so you don’t end up wasting your time on a book that has already been written and is well-established.
Don’t be discouraged if there is already a book out there that covers your topic. Most genres have multiple guides with similar topics (self-help books are a great example—consider how many books exist on relationships!). With millions of readers in the world, there is always room for another guide as long as it takes a different approach.