August Wilson has already given the American theater such spell-binding plays about the black experience in 20th-century America as ''Ma Rainey's Black Bottom, Joe Turner's Come and Gone, '' and the Pulitzer Prize-winning, ''Fences.'' In his second Pulitzer Prize-winner, ''The Piano Lesson, '' Wilson has fashioned his most haunting and dramatic work yet. At the heart of the play stands the ornately carved upright piano which, as the Charles family's prized, hard-won possession, has been gathering dust in the parlor of Berniece Charles's Pittsburgh home. When Boy Willie, Berniece's exuberant brother, bursts into her life with his dream of buying the same Mississippi land that his family had worked as slaves, he plans to sell their antique piano for the hard cash he needs to stake his future. But Berniece refuses to sell, clinging to the piano as a reminder of the history that is their family legacy. This dilemma is the real ''piano lesson, '' reminding us that blacks are often deprived both of the symbols of their past and of opportunity in the present.