The easiest books to write are the ones you already know.
Take a moment right now to write down all the jobs you have had and currently have. Next, write down all the sports you have played, the games you have played, and the clubs you have been a part of. Finally, write down your hobbies, places you have visited, where you live, and things you do on a daily or almost daily basis.
When you are finished with your list, you will have a list of potential book ideas that you can write with very little research.
For instance, let's say that one of your past jobs involved customer service for a credit card company. With this knowledge, you can write books on how to provide the best customer service, how to train customer service professionals, and how to break into the customer service industry.
Let's say you played soccer all through school nearly 20 years ago. You already know the game. You probably also have a good idea on what makes a good soccer coach. Taking this knowledge, you can write a variety of books on soccer for children, teens, and parents of soccer players.
Never discard what you already know and have experienced. That knowledge can be quickly and easily turned into a book that will be very easy for you to write.