Another Patricia Cornwell appears, and it's a pretty safe bet that At Risk will storm its way to the top of the best-selling charts. Cornwell has been at the top of the American crime writing tree for so long now that her position is virtually unassailable--even when she produces the odd misfire (which he has been known to do). Fortunately, At Risk can be counted among her successes, though its brevity may disappoint some. The…