Helen Oyeyemi penned her first book, The Icarus Girl, at 19. Now on novel number four, the frustratingly precocious writer has cooked up Mr. Fox, a story about a literary-world love triangle that’s fleshed out with folktales and reads like a short-story collection. The presentation is scattershot, and the characters sometimes too self-consciously cute, but there’s a bloody beating heart underneath all the postmodern playfulness. As with the hero of Woody Allen’s story “The Kugelmass Episode,” St. John Fox’s love is real, but its object isn’t. The charming, roguish 1930s novelist doesn’t seem especially ...