Arthur Agatston is an unlikely diet guru, but that's exactly what he's become. A nerdy Miami cardiologist who cares way more about lipids than he does about flat abs, his new book, "The South Beach Diet" (Rodale. $24.95), debuted at No. 3 on The New York Times best-seller list last week, just behind "Atkins for Life" by Dr. Robert C. Atkins. Readers may be drawn to the name--who wouldn't want to look like a babe on South Beach?--but the real value of the book is its sound nutritional advice. It retains the best part of the Atkins regime--meat--while losing the tenet that all carbs should be avoided. Instead, Agatston encourages a well-balanced diet that includes plenty of fruit, vegetables and whole grains, plus nuts and healthy oils. Many of the recipes in the book come from Miami chefs. Agatston says the diet works because it's forgiving of minor infractions. "I'm a chocoholic, so I understand falling off the wagon," he says. But with hamburgers--and buns--allowed, who needs to cheat?