Do we really need another "how to" book on advertising? John Hegarty thinks not. The legendary creative leader of London-based BBH believes there are far too many such books, and that they have done us more harm than good. When pressed to name one, he cites David Ogilvy's "Confessions of an Advertising Man." Mr. Hegarty says "His whole thing about rules was such a disservice to creativity. We all know that creativity is about breaking rules."