"Even three or five years ago, there wouldn't have been enough law out there to write this book," says Stefan Presser, coauthor of The Rights of Single People, a new American Civil Liberties Union handbook.

"All the trends we describe -- and the litigation -- happened in the last five."

In the past few years, as the book makes clear, single people have increasingly fought for nontraditional relationships and defended their rights in such areas as housing and employment.

Most of the civil rights legislation, Presser points out, did not provide protection for singles. "Singleness was not perceived then as an immutable characteristic. But penalizing people because they're unmarried should be viewed as invidiously as discriminating on the basis of race, sex or religion."

Among the laws cited by the handbook as discriminatory are those prohibiting cohabitation and fornication.

Cohabitation -- defined as living together as part of an ongoing, nonmarital sexual liaison -- is still illegal in 13 states, including Virginia. Fornication -- heterosexual intercourse between persons who are not husband and wife -- is a crime in the District and 13 states, again including Virginia.

"Those laws represent a way of viewing single persons that our society has to evolve beyond," says Presser, who is legal director for the ACLU in Pennsylvania. "Granted, they may not be heavily enforced -- although for gays, they often are. But landlords throughout the country are making moral judgments out of the same contexts that those laws come from. The laws create an unfavorable mind set."

The handbook cites a New York case in which a woman invited a man with whom she had developed "a close and loving relationship" to move in with her. The landlord claimed this violated the lease, which said the apartment could only be occupied by the woman and members of her immediate family. Arguing that the lease provision violated the law forbidding marital status discrimination, the woman sued. The court of appeals eventually ruled against her, saying that her living arrangements did not comply with her lease.

As a result of the outcry over that case, however, the New York legislature passed a law protecting a tenant's right of cohabitation. The moral, says Presser: "Not until people perceive their singleness as a political rallying point will the laws change."

Another example of discrimination against singles cited by the handbook, this time in Pennsylvania: A librarian fell in love with the library's janitor, who left his wife for her. She became pregnant with his child, and they set up house together. The library fired both, contending that since their relationship was frowned upon by the community, it interfered with the library's ability to serve the public.

"The constitution afforded her no protection," says Presser. "She was an outstanding employe who was never charged with not doing her job."

The Rights of Single People (Bantam, $4.95) was written by Presser, Mitchell Bernard, Ellen Levine and Marianne Stecich -- all Arthur Garfield Hays fellows at New York University Law School -- with the aim of alerting the public to such cases. The book is one in a series of American Civil Liberties Union handbooks, of which ACLU President Norman Dorsen is general editor. Others are The Rights of Authors and Artists; Crime Victims; Employees; Gay People; Indians and Tribes; Prisoners; Teachers; The Critically Ill; Young People and Your Right to Government Information.

"All of the handbooks are written for citizens who have never taken a law school class in their life," says Presser, 32, who is single. He and the other authors, he says, "don't pretend that singles are the new oppressed class. This is simply a guide to developments in the law right now."

Because of the almost total absence of legislation and litigation surrounding the issue, Single People is short -- 113 pages, compared with, for instance, 440 pages for Crime Victims and 312 for Young People. "It's harder to define the extent to which a law is impacting on a person because of his being single," says Presser. "The law tends to impact more directly because one is poor, a woman, a black, or accused of committing a crime."

The handbook covers civil actions, criminal law, housing, employment, personal finances, and child bearing and child rearing. Among examples it gives of discrimination in those areas:

* If two unmarried people are living together and one dies without a will, the survivor cannot recover a portion of the estate under state inheritance laws. Instead, the estate goes to either the deceased's spouse and children or to blood relatives.

* An unmarried couple in which only one partner has an income pays more federal income tax than a married couple in the same situation. Income-splitting, which attributes half of the married couple's income to each spouse, gives them a lower overall tax rate.

* Unmarried couples suffer financially as a result of being ineligible for the estate tax marital deduction and the gift tax marital deduction.

* When people break up after living together for a long time without marriage, one person may be disadvantaged. But without the formalization of that relationship, the law offers no protection.

* Unmarried couples are not eligible for any of the Social Security benefits for survivors, disability or death that typically accrue to spouses. Nor can a woman who had a child with a man she lived with collect Social Security surviving mother's benefits if the man dies.

"When singles have lived together for a long period of time and have exhibited all the caring a marriage at its best does, we feel the survivor should be eligible for all the benefits that accrue to a married couple," says Presser. "We don't see the piece of paper or the blessing of a church or the state as the touchstone for these cases."

The authors of the ACLU handbook are, however, optimistic about changes. "The law is going to be clarified with great rapidity," Presser predicts, "and states are going to move into the category of protecting a single person's rights."

The other side of that development could mean that some people will suddenly find the law making unexpected requirements of them.

"You don't have to get married," says Presser, "but if you enter a situation that looks a lot like marriage, the law is beginning to impose certain obligations on you," including certain responsibilities for support.

One important factor in changing the laws: the increasing number of singles, single parents and cohabiting couples. From 1980 to 1983, according to the Census Bureau, the number of one-parent households increased by 175 percent, one-person households by 173 percent and households of unmarried couples by 331 percent. In that same period, the number of households consisting of married couples with children grew by just 4 percent.

"It's the mark of a balanced society that people in this country are aware they have rights, and that they shouldn't be treated arbitrarily," says Presser. "What would have been acceptable treatment 10 or 20 years ago, isn't anymore."