Poland's struggle for freedom; The Polish August, by Neal Ascherson. New York: The Viking Press. 320 pp. $14. 95.

This is an important book for anyone wanting to understand Poland's brave, exhilarating - and tragic - struggle for greater freedom.

In August 1980 a new generation of Polish workers embarked on what noted British journalist Neal Ascherson calls a ''delicate game.'' They knew they could not overthrow central authority. So they set out to take away the substance of power from the Communist Party and state bureaucracy while leaving them with the form.

''Poland was like an old house living under a preservation order,'' Mr. Ascherson writes. ''The interior could be modernized, even gutted and replaced. But the facade and the roof must stay intact.''

In the months after those heady August days, it became clear that even the roof could not withstand the pressures. The party began to fall apart, unable to reassert its authority. The workers had created a vacuum of power at the top, which they then refused to fill. Extremists on both sides fed stalemate.

In the end the three principal forces in the drama - the government, the Roman Catholic Church, and the free trade union Solidarity - saw the need of a political coalition, a ''patriotic compromise,'' to avert breakdown of the whole structure. But the leaders of all three, says the author, however much they yearned for such a coalition, were driven by forces they could not control. ''The Polish lava was still flowing,'' he observes. ''Until it came to a halt and began to cool, no 'Centre' built upon it could survive.''

Indeed it did not survive. Ascherson brings his penetrating eyewitness account to an end before the events of late 1981, including the military takeover. But in a postscript he notes that, while Soviet instigation of martial law remains unproved, the Russians do bear the responsibility: By refusing to let the Polish Communist Party share power and try a new interpretation of its leading role, they ''cut off the Party's retreat and made collision inevitable.''

As he has done for many years in his dispatches for the Observer and other newspapers, Ascherson writes here with perception, firsthand knowledge, scholarship, and considerable eloquence. Most important, he puts events in the context of Polish history, tracing the complex and dynamic forces at work both before and after a communist system was imposed on Poland.

He reminds us, for instance, that Poles often had an absolutist approach in their fight for independence. In 1920 Marshal Pilsudski launched an ambitious attack on Russia in order to revive the 16th-century Polish commonwealth and settle Poland's eastern frontier. The Bolsheviks counterattacked. The Poles beat them back in a battle remembered to this day. But, as Ascherson comments, the Polish attack, instead of serving as a warning to Poles to be a little less pretentious about their status in Europe, ''ensured the historical suspicion which Russians entertained about Poles would be inherited by the Soviet Union and its leaders.''

By the end of World War II, however, the Poles seem to have come to terms with the inevitability of geopolitical alliance with the Soviets. The central question became whether it was possible for the communists to govern Poland by consent rather than force.

There were, of course, numerous attempts to reform the abhorrent system imposed on the Poles. Ascherson details the recurring upheavals in which new leaders would come to power, make fresh promises, then gradually revert to authoritarianism.

Yet despite the failure of the Poles to loosen the communist yoke, Ascherson does not lose hope in the nation's resilience. However dark the outlook at the moment, readers may take heart from these words from ''The Polish August'':

''It will be hard to pull down the monument at Gdansk (erected to honor those killed in the worker mutiny of 1970). For a stranger, that is the only certainty about Poland's future. But Polish intellectuals, in their steely way, are also certain that there will never be a retreat back down the path travelled since August 1980. The new perceptions about the nature of a free Poland are a priceless and permanent gain, even if they cannot be used for a generation. If the Russians come, they will have come too late.''