“The Right to Privacy,” by Ellen Alderman and Caroline Kennedy, Knopf. $26.95.

Did you have sex last night? Use the telephone this morning? Send an e-mail before lunch? Get stopped for speeding on the way to the store?

Consider for a moment how often during even a routine day that we expect privacy in our lives, perhaps even take it for granted. Though privacy could well be the most valued of American possessions, it is also, according to a new book co-authored by a Maine lawyer, perhaps the most endangered.

Despite what many believe, privacy is not specifically mentioned in the U.S. Constitution. To a degree, it does appear in the Bill of Rights — the 4th Amendment protections against unlawful search and seizure, for example. The U.S. Supreme Court, too, has recognized certain privacy rights.

“Some people say it’s there in spirit,” said Ellen Alderman of Yarmouth, co-author with Caroline Kennedy of “The Right to Privacy.”

In their book, Alderman and Kennedy pick up where their first work, “In Our Defense: The Bill of Rights in Action,” left off four years ago.

“The Right to Privacy,” which so far has received favorable reviews, is a sort of collection of case studies in privacy law and its application, including where it falls short. Avoiding bone-dry analyses and a reliance on legalese, “The Right to Privacy” allows real people to tell their stories in breakfast-table English.

The idea for the sequel, Alderman said during a recent interview at a downtown Portland cafe, arose out of the promotional tour for “In Our Defense.” At many stops, people asked about issues of privacy ranging from abortion to computers.

Whatever the foe — the police, the press, the workplace, even voyeurism — the common theme is a battle against eroding privacy protections. “It made us realize how vulnerable we are,” she said.

Although their division of labor is invisible in their writing, both Alderman and Kennedy brought certain perspectives to both of their books, particularly “The Right to Privacy.”

As the daughter of JFK and Jacqueline Onassis — one of the most fiercely private women in the world — Kennedy brought with her plenty of personal experience fighting intrusions into her life. Alderman, an Indiana native whose father and brother are lawyers, worked as an entertainment attorney before marrying a Portland lawyer three years ago and settling down in Maine.

The 37-year-old authors met in the back row of a civil rights class at Columbia Law School, and their friendship spawned the books. During the 3 1/2 years it took to research and write “The Right to Privacy,” the project was delayed by both women having babies, Alderman’s move from New York City to Maine, and the death of Kennedy’s mother.

It was, though, worth the wait.

The most quoted incident from “The Right to Privacy” involves Chicago women who were stopped by police for even minor traffic infractions. It seems Chicago had a policy for years that forced jailed women to submit to strip searches and cavity checks. Men were merely patted down.

Then there’s the couple who celebrated their engagement with a night at a motel, only to later discover a viewing hole behind a two-way mirror in their room.

And, of course, there are press invasions: the man who saves President Ford from assassination, only to have the media report, accurately, that he is homosexual; Or the man whose dying moments are recorded by a television crew shadowing an ambulance for the day.

Often, Alderman agreed, the right to privacy involves a thin line between legal protections and simple ethics. Some television producers, for example, were appalled at the thought of filming a death without the permission of the family, while others pointed out that the journalists were never asked to leave, she said.

“The people at the home said, `that’s not the point — nobody asked you in ,’ ” Alderman said.

The authors took pains not to editorialize so that readers could decide an issue for themselves, and yet “The Right to Privacy” contains certain morals to consider. In the workplace, for example, an employee should know their company’s policy regarding the privacy of e-mail and other communications, Alderman said.

As well, Alderman and Kennedy show that cyberlaw hasn’t kept pace with technology. Strangely enough, federal law protects the privacy of video rental lists, but not medical records, she said.

“The law,” Alderman said, “hasn’t caught up with many of these areas.”