To cheat or not to cheat — that is the question.

Broken marriages and illicit affairs have inspired literature since the Greek myths and Shakespeare’s plays, and they never go out of style. No one knows this better than Atlanta author Emily Giffin (“Something Borrowed,” “Love the One You’re With”) who regularly tops the best-seller list with books about cheatin’ hearts that her readers inhale like midnight pints of postcoital Haagen-Dazs.

In “Heart of the Matter,” she ventures into more sacred territory — marriage — and opens the book with a wife’s preemptive prayer for her charmed existence. Out at a romantic anniversary dinner with Nick, her gorgeous husband of seven years, Tessa Russo admits that when she hears about someone’s tragedy, she pictures the “before snapshots” — innocent, ordinary moments before disaster strikes. She imagines “the fragile line separating us all from misfortune.” It’s her way “of safeguarding against an after happening to me.”

Minutes later, Nick’s cellphone rings, signaling the end of her perfect world.

Enter Valerie, a 30-year-old lawyer whose 6-year-old son, Charlie, has been burned in a freak accident, and whom Nick, a pediatric plastic surgeon, will meet for the first time that night. Alternating Valerie’s point of view with Tessa’s, Giffin offers parallel views of what’s going on as Nick begins to neglect his wife and two young children in favor of his new patient and and his “very pretty” — and very single — mother.

Okay, so it’s a beach read. A guilty pleasure for some, hopeless fluff for others. Nevertheless, as “Heart of the Matter” takes its sweet time documenting the inexorable slide toward an affair, the slow-moving sequence of events rings true. Seeing it from two sides builds suspense: Will Valerie’s good sense prevail? Will Nick’s conscience step in? Will Tessa find out — or never know?

Giffin has said she likes to write about flawed characters who make choices we don’t approve of, since it’s what happens in real life — people have lovable quirks and good qualities that offset their bad decisions and go onto the “Plus” list when we’re weighing them on the forgiveness scale.

In the lovable quirks department, Nick just squeaks by. He’s a good surgeon, but he can’t stand to be wrong and his salient quality seems to be disdain for lesser mortals. He doesn’t question his feelings for Valerie when they arise, and the ethics of an affair with a client barely trouble his pretty head. He’s nicer to Charlie than he is to anyone else in the book, but that’s part of the problem.

Valerie falls somewhere in the middle. She fails to imagine the repercussions were she to have an affair with her son’s surgeon, the man who will be responsible for Charlie’s surgery, skin grafts and physical therapy. But her love and fears for the child are beyond question, and to her credit, she’s also eyeing Nick as a potential father.

Because Tessa plays an innocent role, her back story heats things up a little to keep her in the ranks of the flawed. Seven years ago, she abruptly dumped her long-term fiance, Ryan, weeks before their wedding in favor of Nick, a stranger she met on a subway ride — so she’s no poster child for fidelity, even if she acted in the name of love at first sight. Could she turn out to be Nick’s “Ryan”?

Throughout the novel, Giffin contrasts what’s real, serious, and lasting with more fleeting, petty issues. While Valerie mentally reruns the before tape — the moment when she relaxed her “over-protective” control and left her son with careless adults — Tessa battles a growing certainty that the safety of her marriage is under attack. Both have valid concerns. Tessa may not be the victim of what we’d call real “misfortune,” but if Valerie, who is, steals Tessa’s husband, she’s no longer the victim. It’s an intriguing seesaw that deepens an otherwise predictable love triangle.

Look for the moral compass of this story in Charlie, an endearingly brave child who wears his heart on his sleeve, faces his plight with grace and wit, and sets a shining example for the self-absorbed adults around him.

Heart of the Matter

Emily Giffin

St. Martin’s Press, $26.99, pp 384

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