THE COOL WORLD (241 pp.)Warren MillerLittle, Brown ($3.75).
The skinny, 15-year-old girl is well schooled in human relations (she has been a Harlem prostitute for a year) and chemistry (drinking water, she knows, helps the smoker extract that last bit of nourishment from a reefer). But Lu Ann is a little weak in geography. "Now Man," she says, "you aint gassin me you really got an ocean you can get to on the subway?" Duke Custis, a knife-scarred hard case at 14, knows well enough where the Atlantic is, even has a vague notion that Europe lies somewhere beyond Coney Island. But...