Ashok, a journalist, dreams of getting his first novel published while his wife, Parvati, a reluctant engineer who longs to be an artist, can't stop fantasizing about her ex-boyfriend. Maya, the founder of a successful preschool, must cope with the knowledge that her workaholic husband, Veer, never got over his first love, another woman named Maya. Sabeena, a homemaker busy caring for her extended family, waffles on whether to have children, while her husband, Shahzad, a chicken seller turned real estate broker, is hellbent on finding a cure for his infertility.

Elizabeth Flock's "The Heart Is a Shifting Sea: Love and Marriage in Mumbai" is a scrupulous nonfiction examination of these six newlyweds from diverse backgrounds (Marwai Hindu, Tamil Brahmin Hindu and Sunni Muslim) who make their homes in India's booming west coast city of 20 million with "majority Hindu and minority Muslim, wealthy and famished, native and migrant, hopeful young men and bent old women, all brushing up against one another."

Flock, a reporter for "PBS NewsHour," began work on the book in 2008, but after time spent in the U.S. recovering from a car accident, she returned to Mumbai to complete the bulk of her reporting in 2014 and 2015. "For months, I lived, ate, slept, worked and traveled alongside them."

This intimate portrait illuminates how the marriages fluctuate between stability and dissolution. Infidelity, infertility, isolation and illness threaten the very foundations of these unions, and unfulfilled goals and dashed dreams exacerbate the tensions. The couples struggle, at times mightily, to honor their vows. Their honesty and authenticity speak volumes about how much they trusted Flock with their stories.

Among the book's many strengths, Flock abstains from generalizing about India or Indian marriages. Instead, she nimbly captures the interiority of her subjects.

"Shahzad thinks of a beautiful woman he once saw on the street and then another. … He does not think of his wife, Sabeena."

The Heart Is a Shifting Sea, by Elizabeth Flock

Maya recalls the timeline of her affair. "May was the 'Big Bang,' or so they called it — the day when the energy and tension between them led to a kind of explosion, everything out in the open at last."

When Ashok is unappreciative of a pre-wedding gift, Parvati is reminded of the vast differences between him and Joseph, the man she could have married. "She realized Ashok was not the kind of guy who would quote her scenes from movies like 'Up,' the way Joseph had. He was not the kind of husband who would be romantic."

Still, the couples forge ahead, as if tied together by an invisible string. And although they are imperfect people in imperfect marriages, in these resplendent passages, their humanity shines through.

Anjali Enjeti's reviews appear in the Atlanta Journal Constitution, Rewire News and elsewhere. She is a board member of the National Book Critics Circle.

The Heart Is a Shifting Sea
By: Elizabeth Flock.
Publisher: Harper, 358 pages, $27.99.