Macmillan's diaries: novelistic skill plus shrewd political analysis; War Diaries: Politics and War in the Mediterranean January 1943-May 1945, by Harold Macmillan. New York: St. Martin's Press. Illustrated. 804 pp. $29.95.

I think I will tell you that the Foreign Office are rather annoyed at your coming (to London) without permission. . . . I must also tell you that I am very pleased you have come. You are not a servant of the Foreign Office. You are my servant and colleague and you must do whatever you like.m

So said Winston Churchill to Harold Macmillan in June 1944. A year and a half earlier, Sir Winston had sent Macmillan to North Africa as British minister resident, an extraordinary position used only in wartime, which combined the power and authority of Cabinet status with the advantages of being in situ.

Macmillan was initially charged with representing British interests in Free French North Africa vis-a-vis both the Allied Military Forces and the French authorities, which were in the process of evolving, first, into the F.C.N.L. (French Committee for National Liberation), and then, into a provisional government for liberated France. This assignment proved to be a veritable mine field of conflicting egos and interests: General de Gaulle was determined to ward off challenges from others less uncompromising than he, while the United States remained disturbingly attached to Vichyite elements - to say nothing of Roosevelt's intense dislike of de Gaulle. Macmillan's American counterpart, Ambassador Robert Murphy, lacked both Macmillan's status and his wily acumen.

These diaries show how Macmillan used his plenipotentiary powers, his astute political judgment, and his keen sense of discretion to further the interests of his government and nation to an extent that far outweighed Britain's actual power.

It is interesting to note that the three major players in the North African sector at this time, Macmillan, de Gaulle, and General Eisenhower, would be the leaders of their respective nations a decade and a half later. One of the many virtues of this book is the insight it provides into the personality conflicts that were to influence the conduct of Big Power diplomacy in the postwar era. Events described in these diaries read like a kind of dress rehearsal for the larger drama these personalities would later act out on the vaster stage of world affairs.

The ''War Diaries'' began as journal letters to Macmillan's wife in England, but the form proved so congenial that he retained his habit of writing her an account of the day's events even on those occasions when she had joined him overseas.

His tone is, thus, light and very personal. Discussion of family matters and lyrical descriptions of scenery are interspersed amid thoughtful political analysis and shrewd character sketches of such figures as Eisenhower, de Gaulle, the King of Italy, Churchill, and Roosevelt.

One of Macmillan's most frequently expressed wishes in these diaries is to have a first-class novel to read. His deep interest in books (including fiction, a genre that is not very popular among many of today's so-called pragmatists) is almost as rare among modern statesmen as it is among professional publishers. Macmillan has been active in the British branch of the publishing house that bears his name for most of his adult life, when not actually holding ministerial office.

The literary judgments expressed in his diary are as strongly felt as his convictions about politics and strategy.

And how many wartime leaders, one wonders, would have thus concluded an account of a day spent flying across Italy and back for a series of meetings in Rome, only to learn that the next day would bring a flight to Athens for a similar round of negotiations:

I have finished ''Our Mutual Friend'' and am short of a good novel at the moment, so I am reading ''Troilus and Cressida,'' in a volume of Shakespeare's tragedies which I have found.

Macmillan's almost novelistic skill in portraying people involved in politics bears comparison with one of his own favorites - Trollope, whose novels afforded him great pleasure and relaxation as he struggled with the myriad political and military problems of his missions. His accounts of his trials and tribulations evince a keen sense of reality, an unfailing pragmatism, and a kind of Trollopean cheerfulness in the face of adversity. His only concession to despair is the recurrent cry scattered throughout these diaries: ''Hinc illae lacrimae.'' But he never gives way to despair; he is too busy coping with reality to allow it to overwhelm him.

Macmillan was indeed Churchill's servant, but only as the prime minister was himself a valiant servant of king and country - and they were alike also in giving superb service to the cause of freedom and democracy.