For any book, or any software, or basically just about any product, think of what would have helped you, but you were unable to find. Then research whether someone else has done it.
As I'm sure you know, it does not HAVE to be a religious book to help people. It could be a book on 100 ways to prepare coconut. If there is an "About the Author" section, you could mention your beliefs there, or even put your testimony in a Foreword.