I ONCE READ A PARABLE about a man who was such a great sculptor that he could carve a grand piano out of a tree stump and then sit down and play it. I've forgotten the moral (if there was one) but the image has stayed with me, and I can think of no living writer to whom it better applies than Harry Mark Petrakis, who has just given us his sixth novel. Like Saroyan before him, Petrakis is a storyteller of immense exuberance, unabashed sentimentality, strong ethnic loyalties (in Petrakis' case, to the Greek-Americans of Chicago), and a bluff approach to the English language that approaches crude lyricism. He writes melodramas and morality plays and, moreover, he gets away with it, largely because his determination to tell a corking good story has a way of sweeping all before him. Nick the Greek, is a case in point; like a piano carved from a stump, it shouldn't work, but it does. It is the oft-told tale of the country bumpkin (from Smyrna) who come to Gomorrah (Chicago), where he wins and loses the love of a good woman (a suffragette) and wins fame and fortune (as a gambler). In other words, the old, old, story. There is only one way to save it -- and, by a happy chance, Petrakis happens to be a virtuoso of this single method. He tells it as though it has never been told before, as though he just this minute thought it up. He sometimes treats the English language the same way, but no matter; in Nick the Greek, as in everything Petrakis writers, there is freshness and charm, but there is also something that has all but vanished in these surly times. It is the sheer delight he takes in his craft, the pleasure of the storytelling itself.