ALMOST from the moment Marshal Tito defied the Kremlin 32 years ago and took his country down the path of national independence, people have been wondering what would happen to Yugoslavia once he left the political stage. Would his heirs sustain his course? Or would the Russians, who have never reconciled themselves to Yogoslavia's defection, try to take her back into the fold?

By coincidence an excellent book by Duncan Wilson, master of Corpus Christi College at Cambridge, has come out at the time when such questions are no longer academic.

Wilson is a scholar who writes from the unique perspective of one who served not only as British ambassador to Belgrade from 1964 to 1968 but also to Peking (1957 to 1959) and Moscow (1968 to 1971). Added to his deep knowledge of 19th-century Balkan history, such diplomatic background gives impressive weight to his assessments of Yugloslavai's future.

One is struck by the pessimism of his conclusions. Although Yugoslavai is likely to enjoy reasonable stability in the short term, Wilson says, the country faces increasing Soviet pressures Moscow's "medium-term" aim since 1955, he says, has been to see Yugoslavia turn into an obedient satellite -- a goal he expects it to pursue not through an immediate military intervention but rather by exploiting the effects Tito's death is likely to have on the multinational country.

Apart from a strong strategic interest in gaining access to the Mediterranean, the Russians would like to eliminate "a source of political infection in Eastern Europe" as well as a Third World force that has hampered their attempts to harness the nonaligned nations to their "anti-imperialist" cause.

As long as Tito was in charge, Wilson argues, the Russians had reluctantly tolerated his "path to socialism," with its market economy, consumerism, open-borders policy, limited private enterprise, self-management and nonalignment. These policies have turned Yugoslavia into "far the most liberal of Communist countries," he says, one in which "most human rights, in a Western liberal sense of the words, are respected much more widely than elsewhere in Eastern Europe."

After 35 years of Communist rule, Yugoslavia "rest on a fairly solid and adaptable basis of institution and habit, whatever troubles may lie ahead in a combination of economic and 'national' crises." And Wilson ses future troubles, because Tito's death will remove, he says, some Soviet inhibitions. "It is the Soviet Union and the Soviet Union alone which can directly upset the present balance in Yugoslavia."

It was in part Tito's prestige and diplomatic skills and in part what Wilson calls "a sort of equilibrium based on implicit bargains" between Belgrade and Moscow that ensured Yugoslavia's independence. For although he experimented endlessly with Marxist ideas to make them fit his needs, Tito had never intended to relinquish his party's political monopoly. He was the last Communist leader to have actually participated in the 1917 Bolshevik Revolution; some hankering for Mother Russia remained despite his defiant policies.

It was during Wilson's tour as ambassador that Tito carried out his major reforms to decentralize the economy, give home rule to all constituent republics and turn Yugoslavia into a loose federation. Wilson concedes that at the time he did not foresee the upsurge of regional nationalism that nearly broke up not only the country but Tito's party as well.

The new leaders are modern men committed to a modern Yugoslavia. The alternative for them is to have a new round of economic reforms which also mean political reforms, or to pursue the Titoist course. Wilson sees numerous risks in both alternatives. Economic difficulties, he says, leading to a revival of tensions among Yugloslav ethnic groups may produce a disintegrating situation in which some elements would call for "the firm hand" -- a euphemism for the return to a Soviet style system.

If there is a problem with this book, it is that it focuses narrowly and somewhat admiringly on Tito and his colleagues, on their maneuvers at home and abroad, and on the ideological intricacies of it all. Wilson's view is very much ambassadorial except that it is presented with a lucidity and elegance that few of his colleagues can muster. But outside the charmed circle of leaders and ambassadors there are the slow processes which do not make the headlines but which over the years produce real changes in the way people live. In the end such changes may influence Yugoslavia's future more than rational calculations.