He also had a new family. He and Montée met at a party in Hollywood in 1991, when he was 40, divorced with no children. She was 11 years younger, with a 7-year-old daughter, Lexi, from a previous marriage. Montée is from Belgrade and has a highly polished complexion and the kind of thick, open-syllabled accent in English that makes “best” sound like “bast.” (Her uncle is Milan Panic, a pharmaceutical multimillionaire and former prime minister of Yugoslavia.) Karp appeared to her so unassuming that when he told her that he took care of the hosts’ son, “I thought, Oh, he’s their ‘manny,’ ” Montée told me.

In 2000, a famous actress brought in her young son to see Karp. She was accompanied by her baby nurse, a British woman named Tracy Hogg. Karp demonstrated his methods for soothing babies, including swaddling and his theory about the calming reflex. About six months later, he says, he heard that Hogg was working on a manuscript about how to calm a baby. That book, called “Secrets of the Baby Whisperer,” went on to become a best seller later that year: “You have to re-create the womb,” it advises, before going on to recommend swaddling infants tightly. “It had nothing to do with my stuff,” Karp said diplomatically. “But it got publishers interested.” Hogg, who died in 2004, advocated what she called a “structured routine” for a newborn — with precise hours for feeding, “activity,” sleeping and “you” time. Karp got to work on his own book, wanting to document his techniques for calming crying while promoting a more lenient approach. He visited his patients at home in order to test out his theories. “I needed to understand, When does it not work?” he said. “I needed to see it in the wild.” He often worked through the night, reciting bullet-pointed thoughts into dictation software because he was frustrated by the pace of his typing, then drove Lexi to school in the mornings wearing the same outfit as the day before.

Karp’s agent turned down a $450,000 advance for the manuscript, and Karp grew apprehensive that they wouldn’t get a better offer. That morning, Montée gave her husband a pep talk. “Act like you have a million dollars in your backpack,” she told him, a joke that is now part of the family lore. She turned out to be right: Karp ended up securing a $1.1 million advance for “The Happiest Baby on the Block,” along with a sequel for toddlers. Karp describes Montée as the one in the household who “seals the deal.” At Happiest Baby Inc., she is in charge of business strategy and creative direction (“As a co-founder, Ruth, I’m so excited,” she told me more than once, having mastered the marketing technique of first-name repetition).

A segment on “Good Morning America” in 2002 helped cement Karp’s success, and Montée had the idea of bringing the book’s testimonial sections to life by recording Karp’s interactions with his patients on DVD. Their timing couldn’t have been better. “The Happiest Baby on the Block” came out just as parenting literature was undergoing a transition of its own.

For years, the prevailing philosophy on child rearing had been Dr. Benjamin Spock’s rallying call to parents: “Trust yourself. You know more than you think you do.” Spock’s “The Common Sense Book of Baby and Child Care,” which was published in 1946 and sold more than 50 million copies worldwide, was so thoroughly embraced that some women kept copies of his paperback all over the house, including in their glove compartments. Detractors later blamed him for the permissiveness that they argued set the stage for the 1960s counterculture — for children being “Spocked when they should have been spanked.” In the late ’60s, Spock’s involvement in the anti-Vietnam War movement caused some of his readers to turn on him. His sales figures soon dropped by half, though he remains, to this day, the world’s most famous pediatrician.

As Spock lost ground, the field of expert advice became more polarized between a “hard” camp, which stressed discipline and conformity, and a “soft” camp, which believed in the importance of bonding and individuality. The issue of baby sleep was particularly contentious. In 1985, Dr. Richard Ferber, who founded what is now known as the Sleep Center at Boston Children’s Hospital, published a sleep manual so popular that his name became a verb. To “Ferberize” is now synonymous with letting a child cry it out (even though his book doesn’t quite advocate that). Following Ferber’s best seller, child-rearing literature was inundated with sleep guides, each with its own impossibly alluring title — “Twelve Hours Sleep by Twelve Weeks Old,” “The No-Cry Sleep Solution.”