Each passing year deepens our realization of the triumph of man's spirit marked by the survival of the great city of Leningrad under the 900-day siege imposed by Hitler's legions in World War II.

Nothing can diminish the achievement of the men and women who fought on despite hunger, cold, disease, bombs, shells, lack of heat or transportation in a city that seemed given over to death. The story of those days is an epic which will stir human hearts as long as mankind exists on earth.

This narrative has itself come to play a role in the Leningrad drama. Published on the 25th anniversary of the lifting of the siege, it has been printed in translation in almost every country around the world. It has been hailed in America in Europe, and in Asia for its celebration of the extraordinary heroism of the people of Leningrad, whose conduct shines like a beacon in a world which is often murky and not precisely heroic.

Only in one great country has The 900 Days: The Siege of Leningrad not been published. That country is the Soviet Union. True, a Russian-language paperback edition was published-but in the United States. True, there are few citizens of Leningrad who are not aware of The 900 Days: The Siege of Leningrad and tens of thousands of them have read its words and treasure them. Nowhere has The 900 Days: The Siege of Leningrad been read more avidly and with deeper insight and appreciation than in Leningrad. But it has not been published there Instead it was instantly attacked by the official Soviet propaganda agencies. Pravda published a full-page attack, charging that The 900 Days: The Siege of Leningrad besmirched the heroism of Leningrad and demeaned the role of the Communist Party in the city's defense. It was, Pravda declared, one more volley in America's cold-war attack on the Soviet.

By the second week of September Von Leeb's Army Group Nord was driving on Leningrad for the kill. He had moved so headquarters up to Gatchina, and from this front-line observation post he got a fine view of the city. All the grandiose architectural ensembles built by Peter and Catherine and the later Romanovs lay spread before him like a panorama - St. Isaac's, the Admiralty spire, the Fortress of Peter and Paul. The dive-bombing and the great fires started by the 240-mm siege guns near Tosno could be followed with clarity. Von Leeb felt victory within his grasp. The Fuhrer seemed pleased and graciously honored him with awards and congratulations on his sixty-fifth birthday. The aging Field Marshal had every reason to believe that he was on the verge of a success which would crown his earlier achievements in breaking the Maginot Line and occupying the Sudeten. Once Leningrad had been captured, he could look forward to pleasant retirement on his East Prussian estates, basking in glory.

Some years after the war Orbeli wrote a brief essay which he called "About What I Thought During the Days and Nights of the Leningrad Blockade."

His thoughts were down-to-earth: of the thousands of treasures of the Hermitage which lay still in the chambers and cellars, subject to damage from German bombs and shells; of the safety of the priceless works of art sent to the Urals; of his native Armenia and the lands of the Caucasus where he spent his youth, and of the scholars of Leningrad and their dedication to science; of his last conversation with Zhebelev, "of his words, of all the thoughts which he then shared with me, of the great strength of the human spirit, the spirit of a man who in the course of his whole life fulfilled his duty unswervingly - the duty of a scholar, a teacher, a citizen."

The life of the Hermitage now descended to the subterranean chambers. Bomb Shelter No. 3, one of twelve in the great vaults under the palace, was the center of activity. Here people lived, worked, studied and died in darkness under the low ceilings. Here were their cots, row after row; here the plank tables where they huddled, swathed in greatcoats, a tiny "bat" light or candle stub flickering over the books of the scholars, the thin scratch of pens on yellow paper, the ink so close to freezing it had to be warmed by their breath. These were the catacombs - the center, such as it could be, of Leningrad's scholarly life. Here people worked until they died. Each day a few more were dead. With the civilian ration down to 125 grams a day (all the Hermitage was on this minimum ration), Orbeli had found one unexpected resource - the by-product of the interminable delay of the painters, the fierce wrangling in which he had been engaged at the time war broke out.

In preparation for the redecorating a quantity of linseed oil had been purchased for the Hermitage stores. There was also a large supply of paste. These products were edible. The linseed oil was used to fry bits of frozen potatoes, dug out of garden patches on the edge of the city. The paste was used to make a kind of "meat" jelly which became the stand-by of the Hermitage diet.

We live in the 18th year since John Fitzgerald Kennedy's life was brought to an end on a bright November day in Dallas, Texas. The shelves are burdened with reports of official investigations and the endless tomes of inquiring scholars, sensational scribblers and assassination freaks.

When will it end? Not soon. Four days after the assassination, in a memorandum to myself (I was in charge of The New York Times's coverage and inquiry into the Kennedy assassination), I wrote:

''The echoes of this killing will resound down the corridors of our history for years and years and years. It is so strange, so bizarre, so incredible, so susceptible to legend making. It matches Lincoln's assassination and may well have equal public effects.''

The latest addition to the assassination literature, ''Best Evidence'' by David S. Lifton, underscores that observation. Nor will this new entry bring an end to what has become a macabre industry.

Mr. Lifton's work has been introduced by Macmillan with fanfare. The advertisements blazon ''The Coffin Was Empty.'' Review copies come with a handy kit of ready-to-ask questions for television interviewers too busy to read the book. There are charts and photographs to provide the all-important visuals.

Mr. Lifton's basic concern is to show that it was physically possible that Mr. Kennedy's body could have fallen into the hands of unknown conspirators some time between 2:18 P.M. November 22, when it was (supposedly) loaded onto Air Force One in Dallas, and about 8 P.M., when it was officially observed arriving at the Bethesda Naval Hospital morgue, where the autopsy was conducted. During this interval, Mr. Lifton postulates, what he calls ''medical forgery'' could have been perpetrated to change the nature of Kennedy's wounds.

Mr. Lifton does not seem very certain about who might have carried out this complicated caper. In fact, he seems to find himself in a state of creative exhaustion after so many years of burrowing in minutiae for materials with which to concoct his elaborate and sometimes almost incomprehensible timetable of the movements of coffins, coffin guard teams, doctors, Secret Service agents, F.B.I. men, Kennedy people and, of course, the actual body of J.F.K.

But, if I read him right, Mr. Lifton suspects several sinister forces - possibly Lyndon Baines Johnson, possibly the C.I.A., possibly the F.B.I., possibly parties unknown. The only thing of which he seems strongly convinced is that Lee Harvey Oswald was, in his words, ''a patsy.'' Mr. Lifton is willing to concede, I think (these points are not always a model of clarity) that Oswald was in the Texas School Book Depository; that Oswald was equipped with a rifle, though he doesn't believe it was the one Oswald ordered from a mail-order house; and that Oswald may have fired one or more shots from that sixth-floor window. But all that is, to Mr. Lifton's way of reasoning, designed only to set Oswald up as a fall guy so that the actual killers, whom he believes assembled on the ''grassy knoll,'' could make their getaway unsuspected by anyone except Mr. Lifton. As to what may have motivated the killers - forget it. Mr. Lifton is too busy with his timetables, his tape recorders, his interviews and his own personal reactions to bother with that.

The ''real'' assassins - and this is the whole point of Mr. Lifton's 747 pages of dense prose - had to alter Kennedy's body so that the autopsy, the inquest, the medics, the reporters, the investigators, the whole world would believe that the assassin was Oswald and not the sinister forces who caused a ''puff of smoke'' on the ''grassy knoll.'' This was no easy task. But, as Mr. Lifton describes it, the assassins were willing to go to any lengths: manipulation of evidence, use of two or more coffins, exchanges of coffins carried out so rapidly it makes the head spin, squads of undercover conspirators (not one of whom has ever been identified) and possible subversion of security personnel. So it was, he concludes, that the conspirators were able to steal the corpse of J.F.K. from under the eyes of his widow and the Kennedy party and accomplish the ''brain-wound forgery'' that threw us all off the track and caused the Warren Commission and almost everyone except Mr. Lifton to suppose that Oswald fired those bullets.

No says Mr. Lifton. The bullets came from the front, from the grassy knoll where so many critics, as he calls them, think they can see a blurred man with gun in hand in one of the snapshots taken of the area.

Well, I guess you can say that no one before Mr. Lifton has constructed a theory so complicated, so quirky, in such violation of every law of common sense and reason. But that is not to say that his efforts will not be surpassed in the future. So far as I know, Mr. Lifton is the first to advance the ''empty coffin'' notion. But it was bound to come. Years ago, when I was a young reporter in Chicago, I had to produce each year a story to run on April 14, the anniversary of Lincoln's assassination. I well remember one such story, which was headlined ''The Coffin Was Empty.'' I got it from an elderly Lincoln buff who claimed that the accepted Lincoln assassination story was a cover-up, that in fact Lincoln had never been shot, that he had wearied of life in the White House and had stage-managed the scene with John Wilkes Booth. The old man claimed that Lincoln had lived on a farm in some backwater of southern Illinois until well into the 1880's. Seven million people had seen Lincoln's black coffin in the procession that made its way from Washington to Philadelphia to New York and west to Cleveland, Columbus, Indianapolis, Chicago and Springfield. They did not know that the coffin was empty.

So it goes. Probably after the year 2000 the Kennedy assassination books will begin to thin out, but mark my word, 2063 will bring a new spate.

But out of all of this, I suspect, not one tangible piece of new evidence will be established: not one new witness who will say, ''Yes, I saw the killer''; not one man who will say, ''Yes, I helped forge the surgical evidence''; not one new bullet, not one new accomplice, no mastermind, no evidence of involvement of L.B.J., Allen Dulles, J. Edgar Hoover, Fidel Castro, Nikita Khrushchev, the K.G.B., the C.I.A., the K.K.K., oil millionaires, Maoists, fascists or members of the Women's Christian Temperance Union.

The labor of Mr. Lifton is the best evidence for my assertion. He has worked 15 or 16 years. He has interviewed hundreds of persons; he has read, so he says, everything. But he has made no case. He has tried to count all the trees in the forest and prove that others have sometimes identified an ash as a maple or an oak as a willow. But he has given us no vision of the forest equal to the one presented by the ordinary working reporters who threw themselves into the inquiry a few moments after John Kennedy was shot.

But none of this will halt the tide of rumor. Psychologically, we cannot now, and probably never will be able to, accept the mean notion that one social misfit with a mail-order gun could bring an end to the dream of Camelot.