My grown daughter is a devout Christian. I am not. She loved science fiction as a child, but now shuns it as the devil's playground (I exaggerate, but you get my point). She will read CS Lewis, however, and loved Narnia et al. Any suggestions for her for Christmas? Christian fiction or Science Fiction (NOT the Left Behind series or anything like it- that is atrocious stuff)? Screwtape letters - yes. She read that. I'm at a loss.
Try Lewis' other fiction, Out of the Silent Planet, Perlandra, and that Hideous strength, also The Great Divorce and Pilgrim's Regress are not exactly fiction, but are told in story form, there is also the book "Until We Have Faces" by Lewis, but it is not blatantly Christian, and may not be appropriate for her, though you might enjoy it. You could also get C.S. Lewis' indirect mentor George MacDonald. His works are strictly fantasy, but if Lewis learned from him, perhaps she would also. Of course there is always all of Tolkien's work, you probably know that he and Lewis critiqued each other's work as they wrote.
Try Summa Elvetica: A Casuistry of the Elvish Controversy by Theodore Beale. If she liked Narnia and Middle-Earth she will appreciate the depth and complexity of Beale's work. He also wrote the Eternal Warriors series, a three-part fictional series on the war between Heaven and Hell. I own all four books, Beale certainly provokes the reader to think and that is refreshing in Christian fiction these days.
For classic Christian fiction, I'd recommend the New Testament. It's not sci-fi (nothing scientific about it!), and it's not well written, but it's a best-seller. And the best thing about it is that Christian who read the book straight through typically lose their faith, or their interest, by the time they get to Paul's epistle to the Romans.
Another great Christian classic is Pilgrim's Progress, by John Bunyan, which is one of two books (along with the family Bible) that used to be found in every American household, back in the days when America was still a Christian nation.
The Loving Prophet takes the reader to other planets to witness the birth of the planet's Savior, the resurrection of another planet's Savior, and fulfillment of prophecies -- all in the context of Christian fantasy and romantic adventure.