THE NEW UNTOUCHABLES: Immigration and the New World Worker by Nigel Harris (I.B. Tauris: $35; 254 pp.). Immigrants don't compete with native workers for jobs; instead, by taking low-paid, dirty but vital jobs that natives are right to reject, they support high-paid jobs elsewhere in the economy. "The United States employs 1 1/2 times as many janitors as lawyers, accountants, investment bankers, stockbrokers and computer programmers put together." Immigrants contribute more to their host countries than they receive in social benefits. The global economy of the future will require more, not less, immigrant labor, particularly in North America, Europe and Japan, where populations are aging.