Romance novelists play with fire. Not the mere sparks struck in sex scenes, but the real, consuming flames of life-altering emotions. Taboos are particularly incendiary. A heroine who defies the customs that govern our relationships with friends, lovers, fiances, ex-wives and ex-husbands is risking a conflagration.

Many central characters in genre fiction are rogues -- think of all those live-by-their-own-rules detectives in mysteries. But when a romance heroine turns renegade, she violates not just some arcane code of conduct but rules that apply to most of her readers. Reading about their violation is a cross between overhearing gossip and sitting on a jury.

Good Girl No Longer

Rachel, the heroine and narrator of Emily Giffin's Something Borrowed (St. Martin's, $21.95), turns 30 in the novel's first scene, old enough to stand the heat when she sleeps with her best friend's fiance, Dex. "I can't stop thinking about Dex," she tells us. "I know that . . . he will marry Darcy in September. But I am content to live in the moment."

Until she sleeps with Dex, Rachel has followed, in her view, a "Goody Two-shoes path with no deviations. . . . straight As in high school . . . college, graduated magna cum laude," then "straight to law school and to a big firm."

Both Dex and Rachel orbit around the glamorous, "high drama" Darcy. Rachel's Midwestern values (she's from Indiana), her general decency and her honesty keep the reader from hating her as she narrates this story of her forbidden affair with the man already claimed by her oldest friend. Giffin depicts the complex, shifting relationship among the three principals, compressing the histories of Rachel and Darcy, friends since grade school, into the five months between Darcy's engagement and her wedding date.

Will either Rachel or Dex tell Darcy that they have betrayed her? Sleeping with your best friend's boyfriend is a no-no; sleeping with her fiance is taboo -- and a thrill to read about.

Old Flames

In A Love Story (Dutton, $23.95), co-authors Denene Millner and Nick Chiles provide a "he said/she said" narrative, alternating chapters between the heroine's and hero's points of view. Nina is another 30-year-old confident enough to disobey the rules, in this case the one that warns you against adding sex to a 26-year-old friendship. She sleeps with Aaron, her oldest friend on the planet.

He understands the risk: "From the moment I first opened my eyes the morning after . . . I had been grappling with this dread smacking me in the back of the head. Why couldn't we just leave it alone? Wasn't a friendship crafted by the angels enough for us?" Nina has an endless "my Man Must-Have List." One item: "He's got to be tall, chocolate, and fine -- but that's a given." Another: "He's got to be at least four years older than me (you know, to make up for that maturity curve we women have over these dumb little boys)." She is angry at men, and she aims that anger at Aaron after they become lovers. Her direct, clear-eyed tone is matched by his earthy narration, told very much from a male point of view, more raw than most romance language. Prompted by an "I think I'm pregnant" phone call, Nina and Aaron flash back to their childhood together and recount their affair and its difficulties. Their recollections offer compelling access to their emotional states as they struggle to remain friends while becoming lovers.

A Cure for Curses

In Unsettling (Rayo/HarperCollins, $24.95) Lynda Sandoval's heroine, Lucy, must defy the family curse. Her wise old female relatives believe that their family members' first marriages -- all of them -- are doomed to end in divorce. "Get your feet wet with this [first] one" is the Olivera family motto. "Lots of other men out there to marry for good." At her wedding shower, Lucy's Tia Manda gives her not one but two toasters. "For the split," she explains, "to make things easier on both of you when . . . you know." Lucy, 38 and a narcotics officer on the Denver city police force, can face drug dealers but not her own wedding reception -- she bolts, fearing the curse will destroy her marriage to her beloved Ruben.

Along with her oldest friends, Mercedes, Cristina and Annette, she takes off on a road trip to New Mexico, in search of a curandera, or medicine woman, who can change her life. Lucy and her three friends face, in turn, their own dark fears: a family curse, kleptomania, remaining an unfeeling boss and loss of identity amid the duties of motherhood. In classic road-trip fashion, each confronts her fears in a sometimes panic-stricken, often hilarious trek.

Scenes from a Divorce

Maddy Green, 37, narrates her own tale about still another taboo: Don't start up again with your ex-husband. As her current boss describes her, Maddy is a "disillusioned art student with an advanced degree, making good with the one marketable skill she learned in college -- PhotoShop." She has indeed moved on since divorcing Jack -- to another city, to a new job, to a new romantic relationship. Jack, the least promising romance hero in this group of books, breaks the "no contact" part of their divorce agreement in the first scene of Brooke Stevens's Kissing Your Ex (New American Library; paperback, $12.95). In a letter he suggests that he and Maddy "go somewhere for a weekend, someplace beautiful that neither of us has ever been to, just to see what it's like to be around each other without all that history of ours." This is a fantasy -- or a nightmare.

Maddy reaches difficult conclusions about Jack: "He always seemed to have one foot out of the relationship," and her profound ambivalence about restarting a marriage that ended in failure is reflected in this novel's quiet, mysterious, sometimes dark tone -- fascinating territory not often covered in this genre.

The Dating Game

It's folly -- if not quite taboo -- to meddle in your friends' love lives. Chris Manby in Getting Personal (Red Dress Ink; paperback, $12.95) introduces two heroines and a hero who also is involved in a courtship. Ruby is secure in her London PR job, and Louisa is a successful fiction editor; it's Martin who most resembles the stereotypical ingenue of bad romances. He is unhappy in a dead-end job selling advertising, while he waits for his real life -- writing fiction -- to begin.

The three friends agree to place personal ads -- for each other -- and then screen the replies -- again, for each other. Their reasoning is British: "While it's not acceptable under any circumstances to blow your own trumpet in this green and pleasant land, it is almost acceptable to play a horn concerto on behalf of your very best mates." This variation on Cyrano de Bergerac has as many unintended consequences as the proverbial circus car has clowns. Ruby, Louisa and Martin find themselves paired with textbook versions of the wrong person -- including, in Martin's case, a dominatrix -- yet this funny novel reaches a satisfying resolution.

Miss Darcy to the Rescue

Some taboos are literary: Mess with Jane Austen at your own peril. Kate Fenton takes this risk in her Vanity and Vexation: A Novel of Pride and Prejudice (Thomas Dunne, $23.95). She rewrites many a Janeite's favorite Austen novel, namely Pride and Prejudice, but updated with a twist: She reverses the gender of the central characters. You don't have to have read Pride and Prejudice -- much less remember it -- to enjoy the rewrite. It works beautifully in its own right. If you do know Austen's novel, you get double the fun.

The heroine -- er, hero -- is Nick, a struggling writer of thrillers, who, like our other heroines, is in his thirties. His wife has divorced him. Our "hero," Mary, is a tall, dark and -- well -- handsome woman, powerful and rich, a movie director who arrives in Nick's Yorkshire town to film -- what else? -- an adaptation of Pride and Prejudice. At the local dance where the first meeting occurs, Mary is every inch the supercilious prig. She and her friends regard the proceedings "with the air of adults at a chimpanzees' tea party, prepared to be indulgent provided it didn't go on too long." The tone is witty, the pace quick and the emotions true to the contemporary characters. Austen's original has great bones, and so does Vanity and Vexation.

These rogue heroines are older than the naive, blushing ingenues once commonplace in mediocre romances. They have lived long enough to acquire complicated emotional histories, and they have the self-awareness to tell their stories with insight. These gripping novels compel the reader to root for heroines -- and to seek out authors -- who disobey the rules. *

Pamela Regis, who teaches at McDaniel College in Maryland, is the author of "A Natural History of the Romance Novel."