Inside a glass-encased gondola climbing more than 9,000 feet, PW sits with Pam and Mary O'Shaughnessy, two sisters who pen legal thrillers together under the name Perri O'Shaughnessy. We span the natural and man-made wonders of the city of South Lake Tahoe: pine trees of countless variety dot the surrounding mountains, the freshwater lake below runs 72 miles in circumference and 1,600 feet in depth, casinos call customers toward the Nevada side to the cathedrals of chance and commerce and, off in the distance, Carson City awaits, where the lush Sierra Nevada meets the parched desert. It is summer, but the many 10,000-foot peaks within view retain their snowy caps, and severe weather is never too far from imagination and memory, where the ill-fated Donner party met its gruesome demise only a few miles northwest of the lake. Abruptly, the gondola stops in mid-ride. Pam looks at the rocky terrain below and says, "We'd roll a long way." Mary wonders if perhaps a rider needed assistance. "Maybe a body got crushed," offers Pam, the older. "Wouldn't it be fun," replies Mary, not really needing any prodding.

It's all in a day's work for the sisters O'Shaughnessy, partners in crime. They call it "research," and with their eighth book, Unfit to Practice, featuring defense attorney/single mom Nina Reilly, published this month by Delacorte and set in Tahoe, they keep their steely eyes focused on their surroundings and scour for possible settings for suspenseful misfortune.

While some might find two sisters plotting murder rather odd,the arrangement suits the O'Shaughnessys just fine, although the collaboration began by accident.

Spend time with any siblings and they are bound to talk about each other's nature and offer up their version of the family truths, and the O'Shaughnessy girls are no different. Both started writing in about the seventh grade (they are a couple of years apart in age), though it was Mary who always knew she'd be published. "Somehow, some way, I just figured I'd stick at it until it happened," Mary says.

"Mary's an optimist," Pam says. "I'm a realist."

"I would say that she gives up too easily," adds Mary, with a laugh and no malice.

It's clear that Mary and Pam O'Shaughnessy respect, admire and enjoy each other, exhibiting the best sororal traits. But did they always get along?

"Always," says Pam.

"We fought like cats and dogs growing up," answers Mary.

"We did?" says Pam. Eventually they agree that while all four siblings (the O'Shaughnessy brood includes a younger brother and sister) fought wildly as children, they are the best of friends now.

The O'Shaughnessys grew up in tract houses around L.A. and had a childhood not without its own mysteries. The family moved every year, but neither sister knows why. "Mom said, 'We had a good reason,' " says Mary. "I always thought it was my parents' way of traveling."

Their mother always regarded the study of law as a great way to organize one's thinking. Pam graduated from Harvard Law School and took the California state bar exam at the same time as her mother, who began her own law studies at the age of 48. Pam, who practiced law for over 16 years, at one time teamed up with her mother in Monterey, with brother Patrick (now also a lawyer) as law clerk. "O'Shaughnessy and O'Shaughnessy, everybody got a kick out of it," says Pam. Later, she had her own practice in Tahoe.

At one point, Mary's mother begged her to study law. "I never did what I was supposed to do," she explains. Instead, she majored in English and had a career in developing various multimedia projects. "All I ever wanted to do was write," she adds.

One thing both women acknowledge they inherited from their father is an Irish mastery of blarney. "He had it in spades," says Mary. Even after his death, the family was never quite sure what Mr. O'Shaughnessy did for a living. "He always told us he was a spy," says Pam, though officially he copped to working for the government in aerospace. "He was a crazy Irishman," she adds, with all of us agreeing that can, in fact, be an occupation.

Often the O'Shaughnessy sisters talk over each other, as happens in families, and still somehow manage to hear what the other is saying. In the end, they say that their father was a charming, gregarious man, a talented exaggerator and born storyteller.

"My husband always teases me that in an argument in our family, the person who sounds the most confident is the one who wins," says Mary. "It has nothing to do with any factual basis. And that is certainly something we got from our Dad, the ability to put one over on someone."

"Remember one of his mottoes?" Pam asks Mary. "Always attack, never defend."

With such a family dictum, it might seem strange that Pam and Mary O'Shaughnessy's creation, Nina Reilly, is a defense attorney. But as they say, the best defense is a good offense, and Nina is not easily swayed. Throughout the course of eight books, she's been shot, driven off a cliff and nearly made victim of an avalanche (which actually kills her husband). In Unfit to Practice, the legal system itself turns on her.

Pam and Mary discuss Nina much as they talk about their family, by comparison. "She is a very impetuous person and an impulsive person," offers Pam. "She's blunt," observes Mary. "But she is probably more cautious than us," says Pam. "She's more like Mom," adds Mary. "Very private." They say they purposely made Nina, who is petite and brunette, not resemble either one of them physically. Both sisters are tall, a tad older than Nina, who is in her mid-thirties, and have the light coloring of their German-Irish heritage, with Pam more blonde and Mary a redhead. Of course, with the collective imaginations of two beautiful women working for her, Nina's blessed with a pretty good sex life. "We have fun writing about that," says Pam.

Where did the life of Nina Reilly begin? It all goes back to the sisters' individual call to writing and their varied lifestyles. In her mid-thirties, Mary was married with three children. "I was finally ready to write," explains Mary. "But as it turns out, I didn't have things to say. I didn't have a plot."

Meanwhile, Pam, who was practicing law, married and raising a young son, had started a book, but didn't have the time to work on it. One day on the phone, Pam told Mary that she had "the best whodunit that had never been done before," and handed her sister the plot and a few chapters, with her blessing to finish the book.

"I wanted for her to call and say, 'Oh, you finished it,' " says Mary. "Instead, it was, 'You've ruined it. Everything is all wrong. You know nothing about law.' "

Pam insists she was not that mean about it, but apologizes just the same. "She was good, and it was good to see that she was good," she says. The book just needed some work. With her sister's draft in hand, Pam was inspired to take another crack at the story. "I took that sappy English major stuff out and gave it some bones," Pam explains. "And she ripped all of my stuff apart."

The sisters continued like that until they finished a 600-page tome, miraculously with their egos intact. "We were really learning how to write a novel together," says Mary. Without a clue or a contact, the sisters grabbed Writer's Market and started sending out their manuscript. An agent expressed interest, but did nothing. Nearly a year went by, and Pam said to Mary, "Let's write another one. But let's write a blockbuster."

Mary and Pam O'Shaughnessy have not lived together since they were children, but suddenly they found themselves sharing an imaginary world. They seldom write together, opting instead to work in tandem. "Sometimes someone will write a good joke and the other will have a good laugh and say, 'I can make that funnier.' And it just goes on and on," explains Mary. Pam adds, "Many times we'll work scene by scene, passing it on." Obviously, they pay close attention to continuity and admit that they never know precisely where the story is headed until they are done writing and rewriting it.

The tide turned for the sisters O'Shaughnessy in 1994, when they heard from agent Nancy Yost, with Lowenstein Associates in New York, who said she was blown away by their writing but had a few suggestions. So they set to work again and, a few months later, Motion to Suppress, their second manuscript, sold at auction to Delacorte in a matter of days. Now with 7.5 million copies of their seven books in print, and an eighth just joining them, both sisters are happily writing full-time.

But before their first Nina Reilly thriller saw print, they had one more mystery to solve: what to call themselves. Their publisher didn't like the idea of having two names on a book jacket. "They wanted one author, Pam, because she had gone to Harvard Law School and had the most cred," explains Mary. All involved decided it was best for the sisters to come up with a pen name. Thus Perri O'Shaughnessy, a combination of both sisters' first names with a nod to Perry Mason, was born.

"We're sort of the example of people who know nobody and know nothing and managed to get ourselves published," says Mary. Both couldn't be happier with their star treatment by everyone at Delacorte and Dell, especially publisher Irwyn Applebaum. Says Pam: "We have this sense of someone behind the scenes relentlessly trying to support us and lying awake at night thinking of ways to help us. It's a wonderful feeling."

Also wonderful to Pam is the ability to practice law in the world of fiction. "You always get to win your case if you want to," she says. Mary says they want the books to be fun, but that they also want to touch on important issues. "I think we are infuriated when we read the paper, like everybody else, about how things are going," she says. "One of the things you get to do when you write is that you get to rearrange things or comment on things that drive you insane."

While Nina Reilly's future is uncertain, Pam and Mary O'Shaughnessy have no plans to break up their sister act any time soon. "We talked about it," says Mary. "But we don't have time [to write individually], while trying to produce a book a year together." When they are not working on their novels, Pam and Mary write short stories, also under the Perri O'Shaughnessy name. "Mary and I have been able to indulge almost all of our ambitions under Perri," says Pam.

Eight books and counting is an arrangement that works. Pam says she and her sister have what the French call folie à deux, or a craziness of two, i.e., a hysteria usually among teenage girls in which they lose rationality and egg each other on to do some horrible thing. "That's what Mary and I have," says Pam, "but it results in something constructive—imaginary murders." As their mom might say, so long as no one gets hurt.