THERE ARE POETRY BOOKS and poetic books—the first a book with poems in it, the second a book that may or may not have poems in it, but that is in some sense a poem itself.
In much the same way there are religion books and religious books. A religion book is a book with religion in it in the everyday sense of religious ideas, symbols, attitudes, and—if it takes the form of fiction—with characters and settings that have overtly religious associations and implications. There are good religion books like The Scarlet Letter by Nathaniel Hawthorne or Wise Blood by Flannery O'Connor, and there are miserable ones like most of what is called "Christian" fiction.
A religious book may not have any religion as such in it at all, but to read it is in some measure to experience firsthand what a religion book can only tell about. A religion book is a canvas. A religious book is a transparency. With a religious book it is less what we see in it than what we see through it that matters. John Irving's A Prayer for Owen Meany would be an example. Huckleberry Finn would be another.
Writers of religious books tend to achieve most when they are least conscious of doing so. The attempt to be religious is as doomed as the attempt to be poetic. Thus in the writing, as in the reading, a religious book is an act of grace—no less rare, no less precious, no less improbable.