WITH his best-selling Wealth and Poverty, George Gilder has established himself as a beacon in the seldom-lit night of highpopular economics. Like his predecessor, John Kenneth Galbraith, Gilder is a graceful writer. More importantly, Gilder has supplied what a large segment of the reading public surely must have been hoping for: a sophisticated yet readable treatment of the economic calamity of their times, written by someone they sense could meet a businessman without being overwhelmed by a desire to wash himself.