RICHARD SELZER, a surgeon who has published two books prior to this one, is a difficult writer to classify. "Do you ask me why a surgeon writes?" he said in his last book, Mortal Lessons. "I think it is because I wish to be a doctor" - not the words, to be sure, of a man whose thinking is dominated by traditional categories. In his writing, accordingly, he attempts to be neither "scientific" nor "humanistic" nor even, as in the cases of such popular medical writers as Rene Dubos and Lewis Thomas, to be scientific and humanistic - to weave these disparate elements of medicine into a well-patterned whole. It is not, of course, that such elements aren't present in his work; it is rather that they present themselves in a kind of primal, undifferentiated state.

The 24 essays and stories in this collection, ranging from operating-room dramas to agonizing personal portraits to tongue-in-cheek polemics, give the appearance less of having been woven from different threads than of having been ripped out intact from the wholecloth of Selzer's life and experiences. Short, witty and passionate, sometimes a little ragged around the edges, they simply stand there, without justification or the need of it, wonderfully, satisfyingly difficult to classify.

None of which, however, is to say that Selzer's books aren't filled with scientific information; on the contrary, medical terminology is used freely in most of his writings. But the knowledge of the human body, mostly the internal organs of the body, which Selzer imparts, is not so much told as shown - and shown with a richness and unsparingness of detail that is at times hard to "stomach." In the present collection, more so than in Mortal Lessons, such descriptions are placed in tense, dramatic contexts; the result for the person reading them is a series of experiences that may be "learning," but are also quite as much harrowing. Like the patients who go through and sometimes come out of Selzer's surgical procedures, the reader goes through and comes out of his literary ones sometimes weak and gasping for air, feeling, for all his "experience," anything but scientifically informed.

In a similar fashion, social and humanistic issues such as "the ethics of medical intervention" are an integral but unstriking part of Selzer's work. What Selzer presents us with is not a weries of pronouncements on social issues, but enactments of the situations from which such "issues" have sprung in the first place.As in his surgery, he goes beneath the surface; he would like to open up for us an invisible parade of the strange, sometimes awful, sometimes wonderful ways people react to pain, disease, and death.

Such a parade, when brought to light, is not always glorious to behold, as when Selzer describes finding a woman he has recently operated on inside a bathroom with her fist stuck wrist-deep in her abdomen, "reaching...deep inside...[for] her pain." Or when he describes a blind patient with both legs amputated who every day throws his breakfast against the wall and then asks, "without the least irony," for his shoes. The reader stares hard at such spectacles, only to find them, after a while, gaping back at him. Their effect may be, as the jacket copy would have it, "mystical" and "sublime," but they are surely these things only to the extent they are a great many other, less pleasant things first. Pain and joy, the body and the mind, the scientific and the humanistic; in all such pairs of opposites, Selzer refuses to settle for abstractions but probes deep within to their single, sensuous source.

Should Selzer, then, be considered an artist - and not a "medical writer" at all? Certainly comparisons between Selzer and doctor-writers such as Chekhov and William Carlos Williams are tempting. One of the pieces in this collection, "Tillim," does in fact bear some resemblance to Williams' "The Use of Force," dealing with the same theme of the violence inflicted on children by doctors (and through them, by the state and society). In general, however, such comparisons are inappropriate; Selzer is really more of a writer-doctor than a doctor-writer, the end term being the one that is definitive. His base, it would appear, is in the body. The further he strays from it - in essays on religion, in autobiography, in fiction - the duller does his literary knife become, the more jagged do its results appear.

Still, to point out that Richard Selzer is no Chekhov is hardly to deal him a devastating critical blow; it is only in such company that one's estimation of him falls. By any other standard, he soars, leaving the vast majority of what are known as "popular medical writers" still toiling in the trenches of "science" and "humanism" below. His writings, sustained by astonishing imaginative, intellectual, and technical gifts, give one the uncanny sense of covering immense distances within tiny, cellular spaces - as though one were in flight within a universe of the body. Traveling with him through this universe is a sometimes bewildering, sometimes terrifying experience, but we at least have the consolation of knowing that by journey's end we will arrive, as T. S. Eliot said, "where we started" - at the body. CAPTION: Illustration, no caption, Detail from the title page of Andreas Vesalius' "De Humani Corporis Fabrica"