I guess this is more of a writer question than a reader question. In general readers don't really care if a book's Christian or not as long as it's good. I've never met anybody dead set on reading only Christian books.
But there are plenty of Christian writers who are dead set on writing them. After all, if you're Christian you want to be a witness to God and glorify him, and how can you glorify him by writing an apparently unchristian book? That would put us in the box of "secular" and how can that be glorifying to God?
I've been wondering about this for a while now. Since I started writing at fourteen my Mom would always ask me if my book had a "redeeming quality" about it. For a long time I thought she just meant all the characters should be Christian, or at least become Christian in the end, or at least I should spell out the gospel at some point.
Then I thought, why? If a book is Christian, who will read it? Probably only people who already know the gospel. So what good will that do them if I tell them the gospel? I've never heard of anybody being converted by reading a book besides the Bible.
I will admit I don't have much expertise on Christian books. If you rule out Lord of the Rings, Harry Potter and The Chronicles of Narnia then I have only read five. Two of them were from the Mandy Series. The other two were from some wild-west genre similar to The Love Comes softly series. The last one I just finished was The Last Sin Eater which was in fact the only Christian book I have enjoyed.
None of these books edified me. None of them drew me closer to Christ in any way. All of them had a moment of someone converting, and most of them were simply every-day lives with characters facing situations with "God's on my side," or "God'll take care of us." Things like that sound unhelpful enough in real life, but they're downright cheesy in a book. And because books say things like that, they're considered Christian.
I write books. My first book, I'll admit, was in the Christian-convert category. My second book was in the Cheesy-Christian category which is why I never had it published. And now, starting my third book which has absolutely no direct reference to God at all, I have chosen to keep it that way.
As Christians we should make everything we do glorifying to God, but the thing is: are books that are geared to help unbelievers come to faith helpful when unbelievers never read them? Are books that simply shove God into them because the author feels he needs to helpful? What makes a book glorifying to God? I think J.K. Rowling understood that in the Harry Potter series when she kept the reference to salvation to the end, hooking an unbelieving audience so that they would hear what she had to say. And hey, what unbeliever hasn't read Harry Potter?
Take Lord of the Rings. There is no God, technically. Actually there are several "gods" who made the world and who were all made by one God. Technically it is not an allegory. So why is the series considered Christian? Because the book exemplifies a Christian response to difficult problems. Books like that help you think about life, help you apply Christianity to problems the world faces.
I feel more edified reading non-Christian books than Christian books. I guess it's because Christians oftentimes don't think about important issues; they assume that "Jesus died" conquers everything. And it's true. But how? What is everything that Jesus conquers? How should we face those issues with a Christian perspective?
Another book is Notes From Underground. The Russian author (and I'm not going to bother trying to spell his name) was definitely a Christian, but the book never mentions God. In fact, the book is the opposite of mentioning God. It's about a sniveling little nihilist who's trying to avoid fate. Because the book brought up these important issues and made me think about them, I felt it was much more edifying than any "Christian" book I have ever read.
Esther in the Bible never mentions God. The whole book never says anything about him at all. But is that book Christian? Well, it's in the Bible! And it points to God's salvation and protection of his people. Jesus's stories don't always talk about God either, but they show what God's world is like.
I've decided that I can glorify God in my books without even having to mention him. The book itself is Christian because I am Christian; the book is Christian because it is written from a Christian's perspective of God's world. I want my Christian audience to think about the issues of racism, euthanasia and tyranny; I don't want my non-Christian audience to shut down from facing these issues simply because my Characters keep saying "God" when they don't need to.
What my Mom meant by redeeming quality was not that the whole story is totally negligent of the real world and only attached to Christianity by a small thread of "God"s mentioned here and there. The book itself will be redemptive if it points to characteristics of God or of mankind that will encourage us as Christians and make us think about important issues. That's what really makes a book Christian.