One way to define success is that we sell what we write. Professionals don't rely solely on agents, but they know what's going on in the world around them. They're aware of writing trends. The best professionals spot trends before they become trends.
In 1989, I wrote a book for caregivers of loved ones who suffered from Alzheimer's. I wrote two other books for caregivers. They didn't do well because I was too far ahead of the loop. In 2004 I did another series of caregiving books with only slightly better results. The trend had begun and I lectured often on caregiving.
In 2009, I started a series of gift books for caregivers and they've done quite well. I'll continue to write in that field, but it's no longer my focus because there are already so many people out speaking and teaching.
That's what I mean by studying the markets. I co-wrote Don Piper's 90 Minutes in Heaven and it's now in its fifth year on the New York Times' best-seller list. Since then, other books have come out about heaven. In the fall of 2010, two books, both about a child who went to heaven, appeared.
In the summer of 2010, I released my book When a Man You Love Was Abused. So far as I know, it was the first book on the topic aimed at the Christian market by a royalty-paying publisher. The book has done well and I know of two other books on the topic that have gotten a thumbs up because of my book's success.
By contrast, memoirs and autobiographies aren't doing well, unless the subject is a celebrity, and some of them haven't shown strong results, such as Kitty Kelley's bio of Oprah.
Awareness of the market doesn't guarantee sales,
but it does increase your chances of selling.