TThis is the memoir/confessional of a chap born of parents wealthy enough to found a magazine and to establish ''progressive'' schools for youngsters, and prominent enough to break bread with the Roosevelts and the Morgans. Young Michael Straight attends such academies in America and in England, then samples the London School of Economics before entering Trinity College, Cambridge, in 1934. There, amid a gaggle of homosexual and far-left compatriots, he stands out as an articulate student politico. In due course he returns to his homeland (''I barely knew the country''), where he does a stint in the State Department, serves as angel for The New Republic, becomes a pennant-waving New Dealer, and agitates to get Henry Wallace elected president.
Wouldn't such a chap have life by the shirt collar? Don't you believe it. ''Caught up in the current of history and carried out of his depth,'' he anguishes under a sense of guilt for his inherited wealth, carries a dread of offending people, and nourishes ''a deepseated need to love and to be loved.'' Against this familiar scenario from the Cold War era, what does Michael Straight do to expiate all that guilt? He joins the Communist Party of Great Britain ''out of the sense of brotherhood that had opened a new life for me.''
The central significance, therefore, of Straight's ''political memoir'' would seem to be the postponed illumination it casts upon the collective soul of that long-ago elite who, in England, once swore never to fight for kind and country but who, both there and in the States, flocked to the communist-tainted columns doing battle on behalf of ''republican'' Spain.
As a belated witness to those schizophrenic times, ''After Long Silence'' makes significant reading. It is exceedingly well written, the author utilizing his novelistic talents in much reconstructed discourse. As to the book's most publicized passages - the background it affords to the spy scandals that have racked Britain - Mr. Straight makes it all understandable. He almost makes it understandable as to why it took him more than three decades to put the finger on Anthony Blunt as a crucially placed Soviet agent. In so doing he brings many of these old conspirers back to life, especially ''exhibitionist'' Guy Burgess, who emerges as a special devil. Along the way we get a pathetic glimpse of the last days of poet A.E. Housman and an eye-filling portrait of Henry Wallace, ''heading for the land of illusions from which I had come.''
''Transplanted as I was,'' explains Mr. Straight, ''I lacked a sense of loyalty to British or American institutions. . . . I had been uprooted; I was waiting to be reclaimed.'' The resultant apologia analyzing the phases of this rescue mission is a minor masterpiece of self-exculpation. It should rank with Whittaker Chambers's ''Cold Friday'' as the testament of one whom many may choose to label a traitor.