Well, I'm not sure how much Christian fiction you're familiar with, but I can suggest a few things. If you're into good vs. evil type books with a lot of layers, I'd recommend Ted Dekker (Paradise Trilogy, Blink of an Eye, Blessed Child, and others). His books often do involve some romance (not always, which is unusual in a Christian adult writer), but he never allows his romances to get trashy or gooey. And, when he does include romance, it's only as a part of his larger story. His characters tend to be very compelling.
I'd also suggest a lesser-known Christian author, John J. Dwyer, if you're into good historical fiction. He knows his stuff historically to the point that it's hard to separate truth from fiction in his writing: Stonewall and the longer Robert E. Lee chronicle the lives of two Confederate generals and those they came in contact with (for example, Wayne Marley, a Union cavalryman-eventually-turned-congressman). Dwyer shows the gritty side of life without becoming trashy in his writing.
Tricia Goyer's also a pretty good contemporary Christian writer. Her writing is somewhat more romance-centered than either Dekker's or Dwyer's, but that's usually not her sole focus. (She has co-written a novel in which some guy out in Montana buys a wife--but that's the exception, not the rule.) Most of her books are about WWII (From Dust and Ashes, Night Song, Arms of Deliverance) although she has written a great trilogy on the Spanish Civil War. Nothing trashy, and some pretty intense action.
If you're really into something challenging, then I would recommend a Catholic novelist from the early part of the 20th century, Flannery O'Connor. Her works (The Violent Bear It Away, Wise Blood, various short stories) are set in the American South, with grace as her major theme. I've also heard of another contemporary Christian writer (unlike O'Connor, still living) named Wendell Berry, who also writes some fairly challenging material. I haven't read anything of his at this point, but I've heard he's good.
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