Coming upon her success as a media curiosity, the 14-year-old all-time champion salesgirl of Girl Scout cookies here exploits her fame with a manual of selling. As if we hadn't enough of such stuff, here's another litany of sales advice. But one frowns upon being churlish toward a teen-ager with the spunk to lecture millionaires on her techniques, her brevity and conciseness--the touchstone of such books--renders this venture tolerable. Andrews approaches selling with bounce and enthusiasm. Her diktats revolve around kindness and stick-to-itiveness. Her formula for success (besides having the good fortune of living in a busy Manhattan apartment building): ""Experience plus confidence equals great selling."" As in all such manuals, she emphasizes goal setting (they must be reasonable), using time well (which doesn't necessarily mean being a grind), using the five senses to help stimulate sales, and not being an ostritch (in other words, always feeding yourself information about both your product and the competition's). This won't replace many of the other self-help books currently available, but the cutesiness of the concept will probably prod sales and interest.