I keep an eye out for topics, sub-topics, or perspectives on a topic that haven't been covered in existing books. When I find a topic or perspective that hasn't been covered and that I am capable of covering, I go for it!
I first did this when I was working as a market research consultant. I was looking for a way to "make a name for myself" by writing a book. I found a niche aspect of market research that hadn't been covered through existing books - and then wrote a book to fill the void ("Competitive Intelligence Workbook," 2001).