I recently joined a Facebook Book Club started by some people I went to high school with. The first book chosen was one I hadn't heard of before: Life as I Know It. I'm currently about two-thirds of the way through. Although I'd consider it women's fiction, the premise is rooted in fantasy. Briefly, after two women are struck by lightning at the same time, one of them finds herself living both lives, switching back and forth from her own life as a single working girl to a married mother of four kids. For me, this would be enough to call it fantasy, though I can see why it's marketed as women's fiction. I think mixing genres in a given book can enrich the story. However, unless there's a way to indicate that the story fits multiple genres, it can be very difficult for a book to reach all the members of its potential audience. Plus, the differences in genre conventions may confuse or alienate some readers.
Do you read or write books that cross the genre border? If so, do you find them more challenging than stories of a single genre?