Choosing a topic comes from what you know best. For those people who have their own business, write information that comes from your business. That will allow you a unified marketing effort of your booklet and your business at the same time. When your booklet IS your business, write about what you know, from any part of your life experiences. Other people don't know what you know.
Many people tend to minimize their own base of knowledge and take it for granted.
Think about all the things you don't know about and all of a sudden, things take on a different perspective in valuing the knowledge that you do have. Bear in mind that there is also no such thing as a saturated market, in my opinion. After all, look how many cookbooks are published each year, with more coming out all the time. It will always come down to how you market your own unique information. I advise people to create a booklet that is 16-20 inside pages.