By Joan Aiken

Doubleday. 336 pp. $16.95

Joan Aiken is an accomplished teller of tales, and it is impossible to imagine her writing a bad book; she is also extremely good at taking a conventional form and turning it slightly askew. More often than not, this has given her stories a quirky charm. But in "If I Were You," a tale of look-alikes who change places, it has produced a book that is well written, fun to read and curiously empty.

Aiken's book, set in England at the beginning of the 19th century, is the classic story of two people from different places (Alvey from America and Louisa from the north of England) who encounter each other at boarding school and, taking advantage of an extraordinary resemblance, decide to switch places.

It is the pious, humorless Louisa who proposes the swap. Her unreasonable parents have told her that, no, she may not go off to India to convert the heathen, but it occurs to that headstrong miss that if Alvey will head north and assume her role as oldest daughter, she will be free to waft away in the wake of the Lord.

Alvey, being more imaginative than the single-minded Louisa, perceives that it will not be quite that simple, but eventually she agrees. After all, she is an orphan who yearns to write, and a year spent at Birkland Hall amid the quiet of hill and stream will give her a chance to finish her novel, "Wicked Lord Love."

And so she tucks herself into the bosom of a large, contentious family who may or may not have noticed that Louisa has changed more than four years at boarding school would reasonably explain, but who don't particularly care. Even Louisa's mother, Lady Winship, who has borne 14 children and seen six of them die, is more concerned with the fruits of her garden than the fruits of her womb. "You cannot carry a child within you for nine months, and have all that trouble, and then lose it, without feeling the loss and waste. But you have to try to protect yourself against feeling too deeply," she says, explaining her indifference.

One of the side delights of reading Aiken is her brilliance at naming her characters. There is the faithful retainer, Grizel, an irritating teen-ager named Parthenope, and two charming and fey youngsters, Nish and Tot, who sustain each other against the rest of the household so that a visit to their room finds them "wrapped, cocoon-like, in all their blankets (for the room was icy), clamped tightly, head to tail, like zodiacal fish."

Why then, with imaginative characters and happenings ranging from the mysterious drowning of a baby (whose paternity is equally mysterious) to the kidnaping of Tot, does the book feel oddly flat? Perhaps because the twists and turns of the plot seem artificial, put there to distract us from the straightness of the path that leads from the beginning of the book to its end. From the moment the false Louisa alights at Birkland Hall, it is clear that she is so much better than the real Louisa that the fooled family will never let her go. This lack of a central tension makes all the other happenings ornamental, things to decorate the story rather than to advance it.

In addition, Aiken has deliberately done away with one of the traditional elements of this type of novel: the love interest. She flirts with it briefly as Alvey-Louisa feels a flutter for the son and heir. But our heroine quickly recovers, and when she finally does receive a proposal, it is from a man who is appalled to learn that the hand he wants to claim in marriage is the one that penned "Wicked Lord Love." "You could not have written that. How could you possibly? I have heard of the book. I have even read it," he sputters, his sadness at her refusal alleviated by his escape from life with a woman who writes.

Aiken has not been able to resist mocking the way the reader plagues the writer. (Early in the book, as Alvey plans to earn her living by her pen, the author makes the parenthetical observation: "Like all beginning writers, Alvey had not the least conception of the length of time that publishers frequently take to make up their minds about manuscripts.")

In writing a romance in which the heroine takes stock of the men and then chooses the pen, Aiken is obviously enjoying herself in defense of women writers. If only, having abandoned romance, she had put something else in its place to give the novel the tension it lacks. The reviewer is a Washington critic and writer.