Another futile attempt in print to blast through the handsome carapace of cinemactor Sidney Poitier -- a man of unusual intensity, perseverance and forebearance. Poitier, who weathered extreme poverty in his native Bahamas, frustration and humiliation in the South, the ghetto hardship of Harlem in the '40's -- is understandably angered by the Uncle Tom image directed at him by militant blacks (and white critics). He can point to the fact that from No Way Out (1949) to They Call Me Mr. Tibbs (1970) the incidence and variety of black roles bas multiplied -- from yassuh to ""call me sir."" It is certainly conceivable that the impressive presence of Poitier had something to do with it. The author traces the out-island-to-Oscar story with a few pauses (the fuss about ""Porgy"" before and during filming; the Poitier/Diahann Carroll romance; early marriage and late divorce). The most fascinating remark of any week is reported unexplained: Hepburn to Poitier filming Dinner, ""Spencer can see right through you. He knows how you are made."" Unfortunately Hoffman can't and doesn't.