In the early 1990s, Joost Elffers, a successful Dutch book packager, met an American expatriate living in Amsterdam named Gary Goldschneider.

Mr. Goldschneider was a brilliant but scattered polymath who completed his studies at Yale medical school but never practiced medicine and later became a concert pianist. He once played all 32 of Beethoven’s sonatas in one sitting for a crowd at a Philadelphia shopping mall, telling a reporter he was “spiritually guided” to give the concert.

Perhaps unsurprisingly, he was also into astrology.

For years, Mr. Goldschneider had asked people he met for their sun sign and date of birth, assembling a database of personality traits he supposed were natally based. Mr. Elffers, now 72, turned his ideas into a book, “The Secret Language of Birthdays.”

Until then, most astrology books, like “Linda Goodman’s Sun Signs,” told you about yourself only in the broad strokes of the 12 zodiac signs: Sagittarians are purportedly free spirits, Scorpios are passionate, and so on.

But now, each day of the year was branded with highly specific information about character traits. “The Secret Language of Birthdays” tapped into something even older than the practice of astrology: human vanity (and suggestibility).

“It’s the book for the ultimate narcissist,” said Megan Newman, the vice president and publisher of Avery, the imprint of Penguin Random House that publishes the book. “You can look up yourself, the ex-boyfriend you’re still stalking on Facebook, your mother-in-law.”

This year is the 25th anniversary of the birthday book, as it’s referred to by fans, though its publisher isn’t printing a special edition. Aron Goldschneider, Gary Goldschneider’s son and the book’s editor, was unaware of the occasion until reminded recently. And of course anniversaries don’t mean much anyway, right?

Still, like the Bible, the birthday book is always there, selling 1.5 million copies and counting, according to the publisher, with little promotion (it remains a No. 1 best seller in the Numerology category on Amazon), living for years on home library shelves and popping up randomly in the culture.

In a 2001 New Yorker profile of Jim Carrey, it was revealed that the actor and spiritual seeker (born Jan. 17, “The Day of the Heavyweight”) brought the birthday book to film sets to entertain the cast and crew.

More recently, Alexa Chung, the British TV host, model and fashion designer (Nov. 5, “The Day of Actuality”), gushed to a writer from Harper’s Bazaar that the 365 detailed personality profiles are “so spot on, it’s insane.”

Ms. Newman said, “We’re still selling tens of thousands of copies a year,” and she expects the book to have “a renaissance,” because of the interest among millennials in astrology, the secret lives of plants and other New Age subjects.

Stevie Anderson, 30, a host of the astrology podcast “What’s Your Sign?,” said the birthday book was her first introduction to astrology as a child back in the late 1990s.

“My mom, who is the most Virgo person on the planet, always had the birthday book out,” she said. “It was a talking point. Any guest who came over, she would open up the book and talk about their day.”

Ms. Anderson remembers her entry (May 2, “The Day of Human Observation”) as being “very on the nose about how I felt about myself: You’re fun and productive, but also a dictator, basically.” As a teenager, she would read aloud from the book during sleepovers with her girlfriends to better understand their personalities.

I have similarly used the birthday book as an interpersonal reference guide ever since a college buddy (and fellow Libra) gave me a copy 20 years ago. Though otherwise a fact-based person, I have a soft spot for the pseudoscience of astrology.

And over the years, I have looked up friends, relatives, romantic partners, my wife and son, as though gleaning some truth about their deepest traits.

Certain sentences from my own entry (Sept. 29, “The Day of the Charged Reactor”) rattle around my head. Reading that Sept. 29 people “seem to have difficulty getting it together” is one reason I spent my 20s laser focused on my career.

The parting meditation for my day reads, “What do you see when you live at the center of the cyclone?” — an ominous phrase I’ve been trying to parse for two decades.

If the birthday book is entertainment, not provable fact, its form nevertheless lends it a degree of authority. At 832 pages, it has the heft of an encyclopedia, and it’s designed like a reference book, down to the formal serif typeface.

Each day of the year gets a two-page spread chock-full of information, including 20 notable people born on that day, the position of the sun’s transit in the sky, the sign and corresponding element (fire, earth, air and water).

The design was Mr. Elffers’s masterstroke. “If you have a book of fake news, you have to go for a very conservative typography,” he said. “So I imitated the layout of the American Heritage Dictionary. Because that is the truth.”

Mr. Elffers does not believe in astrology. He said his rational Dutch upbringing prevents such magical thinking. When I told him over lunch near his home in Greenwich Village how I had used his book to gain insight into loved ones, he responded bluntly.

“For me, I find that horrifying,” he said. “And if you stress another time how much you love the book, I will defriend you and never see you again.”

And yet, when Mr. Elffers was asked about his own birthday (Nov. 21, “The Day of Elegance”), he started talking like it was the dawning of the Age of Aquarius. “I’m a cusp. And Gary has a special place for the cusp, because that’s the mixture of two and therefore there’s a lot of turmoil and creativity.”

“The trouble with a Scorpio is they have no interest in any other sign,” he went on about himself. “They think they’re the only one.”

The real magic of the birthday book is in the writing. Horoscopes aren’t a form known for their literary quality, but each of Mr. Goldschneider’s personality profiles is incisive, direct and specific, yet with the necessary mystery.

Until recently, Mr. Goldschneider lived in a Dutch nursing home in increasingly poor health; he died earlier this month at age 80. It was Aron Goldschneider who revised his father’s writing until it was “crystal clear,” Mr. Elffers said.

Aron, 56, said the basic content came from his father, but he added complexity to the character sketches and balanced those that were too unflattering, filling out the positive side.

“My father would write a page one day because of his experience with three people — people he didn’t even like — that were born on a certain day,” said Aron, a lawyer who lives in Philadelphia. “Yoost would say, ‘Gary, this is not a blessing.’”

Like Mr. Elffers, Aron does not believe in astrology. But he has heard from countless people over the years, he said, who have marveled at the way the birthday book gets to the heart of their favorite subject: themselves.

And he admitted that his own entry (Oct. 31, “The Day of Attentiveness”) rings true. Perhaps because his father wrote it.