Betty Ballantine, half of a groundbreaking husband-and-wife publishing team that helped invent the modern paperback and vastly expand the market for science fiction and other genres through such blockbusters as “The Hobbit” and “Fahrenheit 451,” died Feb. 12 at her home in Bearsville, N.Y. She was 99.

Her granddaughter Katharyn Ballantine confirmed the death but did not provide a specific cause.

The daughter of a British colonial officer, Mrs. Ballantine — then Elizabeth Jones — was 20 and attending school in England in 1939 when she married 23-year-old Ian Ballantine, an American at the London School of Economics. Using a $500 wedding gift from her father, the Ballantines started out as importers of Penguin paperbacks from England and founded two enduring imprints: Bantam Books and Ballantine Books, both now part of Penguin Random House.

Paperbacks had existed in the United States since colonial times, but in the 1930s, they were limited mostly to poorly made “pulp” novels. The Ballantines took advantage of new technology in production and distribution and of a clause in copyright law discovered by Ian that waived fees on books from Britain, where high-quality paperbacks were much easier to find. Ian Ballantine vowed to “change the reading habits of America.”

Charging as little as a quarter, they published a wide range of material including reprints of Mark Twain novels and paperbacks of contemporary bestsellers. They helped establish the paperback market for science fiction, westerns and other genres, releasing original works and reprints of works by authors including J.R.R. Tolkien, Arthur C. Clarke and H.P. Lovecraft. They made their books available in drugstores, railroad stations and other nontraditional outlets. They issued some paperbacks simultaneously with the hardcovers, instead of waiting several months or longer.

Their most lucrative publications came in the 1950s and ’60s, when they were running Ballantine Books. Ballantine editor Stanley Kauffmann, who later became the film critic for the New Republic, acquired “Fahrenheit 451,” Ray Bradbury’s dystopian classic that came out in 1953.

Years later, a switchboard operator at Ballantine had been reading a hardcover edition of Tolkien’s “The Hobbit” and recommended it to the Ballantines. They offered Tolkien’s publisher, Houghton Mifflin, $2,500 each for paperback rights to “The Hobbit” and the three “Lord of the Rings” novels.

Houghton Mifflin initially declined but reconsidered when pirated editions of the books began appearing. Rights were granted to Ballantine, which included a warning on the books’ covers that Tolkien would not receive royalties from purchases of unauthorized copies.

“The whole science fiction fraternity got behind the book; this was their meat and drink,” Betty Ballantine recalled, according to Al Silverman’s “The Time of Their Lives,” a publishing history that came out in 2008.

The Ballantines sold their company in the late 1960s, and ended up working at Penguin Random House. Betty Ballantine’s projects included editing Shirley MacLaine’s best-selling “Out On a Limb” and writing a fantasy novel, “The Secret Oceans,” published in 1994. The Ballantines were voted into the Science Fiction Hall of Fame in 2008.

“We really, truly wanted and did publish books that mattered,” Mrs. Ballantine told the science fiction-fantasy magazine Locus in 2002. “And science fiction matters, because it’s of the mind, it predicts, it thinks, it says, ‘Look at what’s happening here. If that’s what’s happening here and now, what’s it going to look like 10 years from now, 50 years from now, or 2,000 years from now?’ It’s a form of magic. Not abracadabra or wizardry. It is the minds of humankind that make this magic.”

Ian Ballantine died in 1995. Their son, Richard Ballantine, a popular cycling writer, died in 2013. The Ballantines had three grandchildren.

Elizabeth Jones was born in India, during the British Raj, on Sept. 25, 1919. She remembered being taught to read by her father at 3 and was so absorbed by Charles Dickens and other authors that she would acknowledge having a hard time understanding that the characters in their books weren’t real. Ian Ballantine was the cousin of one of her classmates in England.

Soon after marrying, Ian and Betty traveled by ship back to his native New York. They established the U.S. division of Penguin Books, and worked from their apartment. In 1945, they founded Bantam Books, then part of Grosset & Dunlap, and went into business for themselves seven years later with Ballantine Books.

One memorable Ballantine release was inspired by a hoax. In 1956, nighttime radio personality Jean Shepherd was telling listeners that they should ask for a new novel called “I, Libertine,” by Frederick R. Ewing. Bestsellers at the time were based in part on requests at bookstores, and demand was so high that “I, Libertine” appeared on some lists.

But, as Shepherd’s fans knew, and the public only later found out, neither book nor author existed. So Ian Ballantine convinced a friend, science fiction writer Theodore Sturgeon, to write — and write quickly — an actual “I, Libertine.” Shepherd, who provided the book’s outline, recalled years later that Sturgeon worked so hard he fell asleep before he finished the manuscript. Betty Ballantine stepped in, handled the last chapter and “I, Libertine” went to print.

— Associated Press