What book would you like to exist if you could buy it today, right now? Is there a book that is missing or impossible to find? Perhaps you have an old out of print book and you would love to see an updated version. You have searched every bookstore and online as well, but still no luck.
Why not write the book for yourself and everyone else out there who shares the same interest as you? If you saw such a book right now and would instantly buy it, chances are there is a market out there for it and others will buy it too. Give some thought to how quickly you would buy your hypothetical book. Would you think about it for awhile, or buy it immediately?
A benefit of writing a book you would like to read is that when you are finished, you will have become an expert on the subject, improving your life in the process. How nice is it to learn how to become an expert and profit after you are done.