DIETIES TO THE contrary notwithstanding, great writers are mortal: this is perhaps their greatest failing. When you've finished that last major Hardy novel, dug out the most forgotten Tolstoy short story, there is no more to look forward to. So it is with Jane Austen, the ultimate literary narcotic for the true believer. Six short novels to be read again and again, outings and conversations to be relived endlessly, the same people and families to be encountered over and over. But, frustratingly, no more.

One of the true believers--a modern writer who prefers to remain anonymous--has tried to rectify this sorry situation by finishing the novel Austen was working on when she died in 1817. Although less than a quarter of Sanditon is the real stuff, the characters and plot set forth in it were enough tc give the rest of the story a veritable Austen contour. Charlotte Heywood is a sensible and clearsighted it quiet country girl staying with Mr. and Mrs. Parker at Sanditon, a new seaside resort on the Sussex coast that Mr. Parker is most anxious to promote. There she falls in love, gets involved in an unforeseen elopement (not her own) and escapes a farcical abduction.

The typical Austen characters, tirelessly wrapped up in their own worlds, are there from the start. Mr. Parker is preoccupied with the financial success of Sanditon. Lady Denham, his collaborator in the resorts development, is obsessed with avoiding her numerous poor relations who, she is sure, are out to get her money. Sir Edward Denham, one of these poor relations, recites poetry, inaccurately and inappropriately, trying to turn the head of every young lady around. There is an officious and hypochondriacal set of Parker relatives; there is the beautiful and aloof Clara Brereton: there is the morose and mysterious young man. There are the requisite and indistinguishable Miss Beauforts, "just such young ladies as may be met with in at least one family out of three throughout the kingdom" interested only in captivating a man with a fortune.

THE CHARACTERS are always the meat of an Austen novel, and Sanditon starts off with an ample and promising cast. Arthur Parker, to all appearances a hardy young man, is first encountered huddled by the fire. "We should not have had one at home," he apologizes to Charlotte, "but the sea air is always damp. I am not afraid of anything so much as damp." But "Another Lady," unfortunately, cannot sustain the kind of dialogue in which the characters betray their own follies. Midway through the book, extensive descriptions of Charlotte's growing feelings for her Prince Charming, and a plot that is a little too complicated, rather than Austen's vivid and endlessly amusing characters, have become the focus of attention.

The loss is immeasurable, for with the characters and their dialogue go the essential vitality that is the mark of the real Jane Austen. The immediacy of the people and the situations is something that can't be replaced by the Other Lady's obviously thorough knowledge of late 18th century carriages, clothing and architecture, a knowledge that is a little too ostentatiously displayed. Austen's world--the slow-moving life of the English country gentry--as the only world she knew, is utterly genuine, even universal. "Every neighborhood should have a great lady," she writes early on in Sanditon: it is a truth straight out of her experience, as real as the one that begins Pride and Prejudice--"that a single man in possession of a good fortune must be in want of a wife."

BUT THIS WORLD, reconstructed by an outsider, somehow just doesn't ring true. In the hands of the other lady, Sanditon inevitably becomes something different, more a romance than a novel, more a fairy tale than a quietly satirical exposition of the familiar and the real. "Readers will find here a refreshing change from the violence and general gloom that pervades so much modern fiction." Sanditon's inside leaf tells us indeed they will--Sanditon is certainly entertaining and has the great virtue of ending happily. But they will not find Jane Austen.