LIKE THE LAST CAPE of an archipelago, some of it passed unseen in the dark, The Sum of Things brings to a close Olivia Manning's sequence of six tragicomic novels following the lives of a young English couple from the beginning of the Second World War to its near-end. There will be no more of these beckoning literary promontories, for the author died suddenly last summer, bitter, we are told, at having ben too much neglected. She had reason. How many Americans who read Barbara Pym, Beryl Bainbridge, or Iris Murdoch have ever heard of Olivia Manning? Yet she is one of the most gifted English writers of her generation -- not womam writer, either; for in spite of her sensitive insight into feminine states of mind, she is equally at home in worlds from which men exclude women, and public themes come to her as easily as private ones. If the author of this pair of trilogies were "Anonymous," the sex could not be guessed.

This is a curious accomplishment, partly accounted for by this restrained and civilized writer's ability to penetrate mentalities very unlike her own and to infuse a knowledge of circumstances she could never have participated in, or even observed, with an imaginative life that is vivid, rich and precise (her description of the battle of Alamein in the penultimate volume is an uncanny tour de force). All the same, these novels, to which The Sum of Things draws a meloncholy conclusion, are closely based on her own experiences between 1939 and 1943. They inform her extraordinary evocations of place and mood, and the sense of history, tragically shuffling human lives, that looms disquietingly over the entire work.

In the earlier Balkan trilogy, a single protaonist, the observant, critically-minded Harriet Pringle, wryly observes the maneuvers of her "insatiably gregarious" young husband, Guy, and a large cast of raffish waifs and dissidents washed up around them on one doomed city after another as the war engulfs Europe and North Africa. Spared from military service by poor eyesight and government employment as a lecturer in English literature, Guy lavishes charm and shallow concern on his exotic students in Bucharest, Athens, and finally Alexandria, while ignoring his wife. She appreciates his bland, even arrogant lack of self-interest, but by the time they have been evacuated to Egypt, the Levant trilogy, she has begun to wilt and sicken under his neglect. When this last novel opens, their marriage is on the rocks.

But the Levant trilogy has another protagonist, and through his pristine sensibilities the boredom, terror, anger, pride, and intense attachments of warfare are preternaturally evoke, and the horizonless desert, swimming in heat, disfigured by shattered military hardware, becomes a sinister presence. Simon Boulderstone is just 20, an innocent cutting his milk teeth in Montgomery's army. On leave in Cairo, his path first crosses Harriet's, but it is Guy who takes Simon under his wing.

For Harriet has left Guy. To be rid of her reproachful sadness, he sends her home to England on an evacuation ship loaded with women and children. At the last moment, she veers off the unwelcome course he has set her (he was too busy to see her off), and hitches a ride in a supply truck headed for Damascus. For the next few months she meanders about the Levant, often alone and short of funds, never knowing that the ship she was to board had been torpedoed and sunk, and that in Egypt her husband believes her dead -- though the fact "that others grieved for her in some ways lightened his own burden and the debt he owed her."

Simon too has escaped death, though not so unawares. A mine has blown up his jeep and left him paralyzed from the waist down, He lies in a hospital, hectically relieved to be alive, later stunned when the realizes he may never walk again or even make love. Guy finds in Simon a ready-made subject for his compulsive need to establish dependency, a sort of substitute for he "dead" Harriet. He is all but aggrieved when Simon begins to recover sensation -- "a stirring in his left upper leg as though an insect were crawling under his skin"; for he wishes to prepare him for life as a cripple by "training his mind," while Simon strugles forward with physiotherapy.

Harriet, in Damascus, has been offered protection by a courtly Syrian. This she rejects: "Good heavens, no. I don't need protection. I'm an Englishwoman." But it is a relief when finally, almost broke, she runs into a Cairo friend who has left her own husband and run off with someone else's -- one of the more raffish British hangers-on at the Anglo-Egyptian Union. Happily, the friend Angela is rich and generous; her lover, the once-despised Castlebar, begins to seem admirable to Harriet: he is kind, and "she was beginning to realize kindness counted for more than wit and intelligence."

Harriet moves with them from one posh Levantine hotel to another until at last she hears the news of her death. For all the pain she has suffered at being excluded from Guy's "widely admired and distributed generosity," she is filled with pity: "Poor Guy thinks I'm dead!" They are restored to each other at a Cairo party. He sobs loudly with relieved joy, but then must rush off to resume business as usual, good deeds to strangers. Harriet has returned -- that is enough for him; life will go on.

But not for Angela and Castlebar. He falls ill and absurdly dies in a hospital from which his grief-stricken mistress is barred, while his monstrous wife, a malignly comic figure, guards the sickroom door. Simon, on the other hand, has recovered the use of his legs and will be sent back to active duty.No longer a helpless innocent, he now needs no one -- the inadequacy that had attracted Guy to him was only accidental. Even Harriet's need for Guy has altered. Why their marriage should survive at all the reader may wonder; that it does can only be explained by Guy's refusal to acknowledge human failing, and Harriet's bleak acceptance of things as they are: "I am imperfect world, marriage was a matter of making do with what one had chosen."

Loss, separation and death, which haunt this last volume, united with the knowledge that there can be no sequel, make it more tragic than comic, though Olivia Manning's humor flickers like heat lightning through every passage. Her restrained compassion redeems all but the most ignoble and cruel of her scrupulously observed characters -- even Guy -- for she is a stern and subtle moralist.

Nobody has written better about World War II -- the feel of fighting it and its dislocating effects on ordinary, undistinguished lives. These trilogies might be made more accessible by shaking out the repetition entailed in the serial form the author chose and condensing them to make two or three volumes. Published as a trimmer whole, this richly absorbing and affecting monument to a most individual talent could find the larger American audience it deserves. Its projected presentation by Masterpiece Theatre should gurantee that.