Almost 20 years ago, Truman Capote, defending his excursion into journalism in his ''nonfiction novel'' ''In Cold Blood,'' said that too many American writers had lost touch with the world beyond their own imaginations, had become ''enraptured by their navels, and confined by a view that ends with their own toes.''
Capote could not have had Philip Roth in mind at the time. Roth had published only ''Goodbye Columbus'' and ''Letting Go'' when ''In Cold Blood'' appeared, and both books stood solidly in the tradition of American realism. Roth soon moved into new areas, however - sexual-psychological drama in ''Portnoy's Complaint'' (1969), political satire in ''Our Gang'' (1971), and unbridled fantasy in ''The Breast'' (1972) and ''The Great American Novel'' (1973) - and in his three latest novels he has, to some extent, been gazing at his own navel and toes.
''The Anatomy Lesson'' completes Roth's trilogy about the Jewish writer in America by taking up the story of Nathan Zuckerman, the hero of ''The Ghost Writer'' (1979), and ''Zuckerman Unbound'' (1981), as he turns 40, still somewhat battered by the storm of publicity kicked up by his best-selling novel ''Carnovsky.'' But Roth's novelist has more debilitating problems now: a severe and mysterious pain in his neck and shoulders that makes it impossible for him to work, a paralysis that also suggests his deeply troubling fear that he may be finished as an artist.
Zuckerman tries everything - from pills and alcohol to neck braces and women - to end the physical pain and restore his literary confidence, but to no avail. He even decides to give up writing and try to recover the lost idealism of his undergraduate days by applying to medical school in Chicago, and the novel ends in a blizzard-bound Chicago cemetery with a drunken and drugged Zuckerman trying to strangle the aged father of an undergraduate friend - one in a series of Jewish father figures that Zuckerman struggles with throughout the trilogy.
The inevitable difficulty of writing three novels about an obsessively introspective writer can perhaps best be seen by comparing Roth's books about Nathan Zuckerman with another major trilogy completed recently - John Updike's ''Rabbit Run,'' ''Rabbit Redux,'' and ''Rabbit is Rich.'' Whereas Updike's novels gain in richness and complexity as they follow their thoroughly middle-class hero, Harry Angstrom, from his young adulthood in the 1950s to his middle-aged complacency in the 1970s, Roth's trilogy, which covers the same decades and roughly the same span in his protagonist's life, falls victim to something like the law of diminishing returns.
The best of the three Zuckerman novels is easily the first, a remarkably taut and subtle book with well-developed characters and an intricate overlay of plots , subplots, flashbacks, and fantasies. ''Zuckerman Unbound'' and ''The Anatomy Lesson'' are much baggier novels that tend to lose their way amid their herd's self-absorbed outpourings of rage and regret. And indeed it may be because one protagonist looks so insistently inward and one so engagingly outward that the world of Nathan Zuckerman the novelist is not, as interesting as the world of Harry Angstrom the used-car salesman.
Nonetheless, Roth's trilogy, taken as a whole, is a significant achievement - a comic, human, and at times profound exploration of a whole range of feelings and attitudes that characterize contemporary America, from the alienation and frustration of the artist to the painful divisions between generations, and on to the disappearance of a way of life and set of values lost somewhere in the shuffle between the 1950s and the 1970s.
For one thing, few American writers have been as successful as Roth is in describing what it is like to be a writer. ''I turn sentences around,'' says E. I. Lonoff, the established author to whom the young Zuckerman makes a pilgrimage in ''The Ghost Writer.'' ''That's my life. I write a sentence and then I turn it around. Then I look at it and I turn it around again. Then I have lunch.''
By the time Zuckerman reaches his middle age, in ''The Anatomy Lesson,'' he has begun to understand what Lonoff meant, the curse of living and working in an all-consuming solitude, of being cut off from a normal, healthy life by the occupational compulsion to see everything as grist for the fiction mill. And it is testimony to Roth's comic genius that he is able to raise such questions by means of a characteristically self-directed irony, as when, in ''The Anatomy Lesson,'' Zuckerman recalls seeing a production of Beckett's ''Waiting for Godot'' and commenting: ''What's so harrowing? It's any writer's ordinary day. Except you don't get Pozzo and Lucky.''
At the end of ''Zuckerman Unbound,'' Zuckerman has his chauffeur drive him through his old neighborhood in Newark. The scene of his childhood and adolescence - the inspiration and subject matter of his fiction - has all but vanished, having been replaced by a black ghetto. As Zuckerman stops in front of a faintly familiar house, a young black man asks him, ''Who you supposed to be?'' And Zuckerman can only answer, ''No one.'' Behind the pain and paralysis that dog Zuckerman through the pages of ''The Anatomy Lesson'' lies this sense of a loss of identity, itself closely related to Zuckerman's realization that, as he says, ''Everything that galvanized him had been extinguished.''
''The Anatomy Lesson'' ends with little hope that Nathan Zuckerman will find something to galvanize him into writing again. As for Philip Roth, he seems to have found sufficient inspiration in the idea of the failure of imagination, of the possibility that the American writer of his generation may have nothing left to say. In doing so, he has said a great deal.