Do not think of Walker Percy's work as Southern literature. I say this not because it isn't Southern literature, but because hardly anybody seems to understand what Southern literature is. Percy, himself, noted this.
Think of Percy's work as good books that deal with the South, but more importantly with people -- with what it means to be human.
The Second Coming is one of my favorite novels. It deals with the existence of God, the fecklessness of modern life, and any number of other banal, overworked subjects that you might find in any other contemporary novel, but they are enlivened by Percy's malicious wit (he called himself malicious, though his doing so was simply an instance of his peculiar malice, which is not really malice, though its sting is the same). The response to the question of God's existence is a toothache.
Percy writes in a straight-forward, ironic manner, but where normal irony is double-voiced, Percy's is triple-voiced. One must always ask oneself if one is really getting the joke even when one is laughing out loud.
Don't think of Percy as a Southern writer because you can't help but shortchange him when you do so. He presents himself with a Southern drawl, and a casual wit, but behind this is incisive social and psychological commentary, and behind this is yet another layer.
The Second Coming is a fine novel -- a good love story if you can stand the fact that the lovers are a mental patient and a horny widower. Percy tells fine jokes, and tosses you on your rear every other page. This is enough, but it is not all. You can enjoy this novel if you just want to be entertained, but if you are willing to look for it, there is an undertone of malice that isn't malice, and yet deeper, a still, small voice.