The problem is that by the time we are able to recognize a trend, it is usually on its way out. This is especially true when you are talking about books. Because a book often takes a year or two from completion to finished product (to say nothing of the several months to a year or more to write it), most trends would be a distant memory by the time the book hits the market.
For that reason, I have always encouraged writers to pay less attention to trends and more attention to their own or their friend’s needs or concerns and write to those instead. If God gives you a burden for a particular subject or audience, that is a greater indicator of direction than is some undefinable trend.
What is more important than topic trends are the industry trends. Reading writer’s newsletters or magazines, belonging to a writer’s e-mail or Internet group, attending conferences, and talking regularly with other writers will help keep you informed on what is happening in the marketplace.