My father is not dead. My father is a book, and books do not die. -- Elie Wiesel, The Testament

If you had just enough time left in your life to write only one letter, who would you write to? What would you say?

Perhaps you would write to your children, your spouse, your parents. Would you pass on your most cherished thoughts and feelings about life? Your most intimate wishes for your loved ones? Would you thank? Forgive? Rebuke? Blame? Try to influence from the grave as you may have in life?

These are some of the questions you would face if you decided to follow an ancient tradition that is enjoying a renewed flowering of interest: the ethical will.

Sometimes called a love letter from the beyond, a gift to the future, a legacy of intangibles, the ethical will is an intensely personal statement to your survivors of your wishes, your feelings, what your life has meant to you.

You may be very specific: how, where and with whom you wish to be buried. One man requested that a Chopin march be played; another asked that his ailing wife not attend his funeral because the strain would be too much.

You may wish to give permission for your spouse to marry again.

You may speak expressly to individuals: to instruct your children on how to conduct their lives -- what wishes you have for their future and religious and ethical precepts you hope will live after you. One woman admonished her four children to share: "If it is only one dollar, you should each get a fourth of it." A man left a fund to pay travel expenses for his family to gather together each spring for a special religious celebration.

"When we think of a will we usually think of a legal document intended to divide up one's property," says Rabbi Jack Riemer of La Jolla, Calif., who with Rabbi Nathaniel Stampfer of Chicago has written Ethical Wills (Schocken Books, 1983, $16.95), a collection of traditional and contemporary documents from which examples here are taken.

"But there is another kind of will," adds Riemer, 55. "Its aim is to bequest a spiritual legacy."

Such wills he calls "windows into the souls of those who write them. They are left behind in the belief that the wisdom acquired in a lifetime is as much a part of the family's legacy as are its material possessions."

The tradition of the ethical will is an ancient Jewish one, stemming originally from biblical injunctions to pass on a way of life to future generations. The practice of writing down ethical directions and wishes for the guidance of children became a cherished custom from about the 12th century onward. Many ethical wills by distinguished rabbis and scholars throughout Europe exist from that date on.

The custom gradually fell into disuse by the early decades of this century. It is only now being revived on a wide scale that includes men and women from all religious backgrounds. "We are finding that psychologists, ministers and professors of ethics and theology all over the country are using this idea to help people clarify their values," says Stampfer, who with Riemer has been invited to appear on numerous television shows and at workshops around the country. Their book is now in its second printing.

A recent will is in the form of a letter to her children written by a young mother dying of cancer. "What is important," she wrote, "is to make each day good, and not to say 'tomorrow' or 'in the future it will be better' . . . I want you to know how much I love you . . . You mustn't be bound by what I would like for you. I expect you to go your own way as good people, the best way you know how."

A grandmother in Chicago wrote her last wishes following an illness, requesting that if she died before her bachelor brother, her children should "see to it that he has a place to sleep, with something to eat at all times."

An elderly gentleman pleaded that if he die before his wife, his children should "call Mom if possible every day -- visit her once a week -- dine with her once a week -- call her and tell her what you want for dinner -- make her laugh."

A brief will written hastily by a young soldier fighting in the 1948 Israeli war of independence "to be opened only after I die" specified that any money in his estate should be spent for guns for his compatriots. Sometimes called a love letter from the beyond, a gift to the future, a legacy of intangibles, the ethical will is an intensely personal statement.

In a far different vein is the will left by humorist Sam Levenson. In his "Ethical Will and Testament to his Grandchildren and to Children Everywhere," he left his "unpaid debts. They are my greatest assets. Everything I own -- I owe.

"I leave you the years," he added, "I should like to have lived that I might possibly see whether your generation will bring more love and peace to the world than ours did."

You need not be a professional writer to compose an ethical will. "One does not need big words or fancy quotes or a formal style," says Riemer. "Spelling mistakes, grammatical errors are not in the end the things that matter most."

He quotes a famous saying from the Talmud: "Words that come from the heart enter the heart."

And there is no need to wait until old age, or a medical crisis, to write an ethical will. Rabbi Stampfer, 59, who teaches a course in ethics, regularly requires his young students to compose a will addressed to their parents. "They are forced," he says, "to look over their lives and learn what they want from them."

Elizabeth David, 49, of Sudbury, Mass., mother of five, has led workshops on ethical wills throughout New England and is now teaching hospice volunteers to compose their own wills so they can help patients do the same. "It is," she says, "a valuable experience no matter what stage of life you are in.

"For me personally," she adds, "it was transformative. It made me want to say the important things to people now, not wait till I am dead."

It is not easy to write an ethical will. "A person who writes his will sees death before his eyes," wrote one man.

"Many boxes of tissues were on the tables," says workshop participant Rosie Rosenzweig, 48, mother of three, of Wayland, Mass. After the workshop, Rosenzweig wrote: "I could feel us reaching out for those feelings long hidden even from ourselves. We uncovered our most intimate selves . . . our only true legacy . . . we wrote . . . and wiped our moistened eyes."

An ethical will can do harm as well as good. "The danger is that one may be tempted to be petty or vindictive," observes Riemer. "But most I have read are loving."

A will may be changed over time. "It isn't written in concrete," advises Stampfer. Adds David, "It should be updated every few years. What I would write to a 13-year-old is not what I would write to an 8-year-old."

Bel Kaufman, 72, author of Up the Down Staircase, says that as the recipient of an ethical will -- left by her grandfather Sholom Aleichem, whose stories became the musical "Fiddler on the Roof" -- she treasures the "remarkable legacy of love and laughter in the face of adversity" he left her. Following his express wish as stated in his will, she gathers about 100 family and friends at her New York home to read his stories each year. "We laugh," she says, "and cry together."

Josephine Benditzon, 42, of Skokie, Ill., mother of two children and a librarian, treasures her grandmother's final wishes, which were scribbled on a piece of paper following an illness. "I cried when I read it, yet with her words I feel so close to her, so thrilled she left her thoughts behind.

"In a way, they keep her alive."