A book isn’t created in God’s image, can’t commit sin, exercise free will, or be saved and sanctified. But it can portray the results of sin, describe grace, and be used by God.
A book is paper printed on and bound together into a volume for reading. Or a digital file stored in a computer database or hand-held device for reading on a screen. Or, perhaps, a recording of written material into sound bytes for listening. Books come in many forms. And there’s nothing intrinsically “Christian” about the forms.
The person who envisions, experiences, writes, edits, and/or publishes a book, however, may certainly be Christian. If a believer in—and follower of—Jesus Christ writes a book, I believe that will be a “Christian book.” That book will be written from a mind that is being renewed, a mind that seeks to view the world as the Bible views it and as the living Word, Jesus, views it; it will be written out of a heart that is stone made flesh, set on and responsive to the Redeemer-King; written from a soul that is being restored according to the holy Creator’s plan.
I haven’t found a perfect book yet, or a perfect person. But I’ve known Christians whose lives ring true, and I’ve read books that ring true.