One of the major genres, books concerning religion and spirituality are as old as the Bible and will likely be written so long as humans seek something beyond the material and ordinary. Although many of these books are written by experts, you don't have to be an authority to write a religious or spiritual book if you have a unique personal experience or a vision that appeals to a segment of the public. If you choose this genre, you'll have to master its writing techniques and recurrent inspirational style.