This is barbecue at its most extreme–grilling in the embers. It may seem radical–even revolutionary, but this was probably the first form of barbecue–pioneered a half million years ago, when man became the first animal to cook. Roast the sweet potatoes on the embers until black on all sides. The flesh will be incredibly sweet and smoky. The complete recipe can be found on page 390 in How to Grill.
By the way, you can cook corn, onions, potatoes, even steak in this manner. For a way cool New York strip recipe, check out the Dirty Steak recipe in the Beer Can Chicken book (page 194).