Duke Univ. 294 pp. $24.95  Why isn't Nicorette gum a street drug? The Food and Drug Administration considers nicotine highly addictive. Tobacco companies seem to share this view when they manipulate the level of nicotine in cigarettes. But the gum, which packs a goodly dose of nicotine, appeals to almost no one. While we're at it, if nicotine dependence is what stands in the way of quitting, why do patched smokers  --  their brains well-supplied with the substance  --  still crave the next drag?