When Disney released "The Black Cauldron" in 1985, the fantasy flick was 10 years in the works and reportedly the most expensive animated movie ever made at that point. The reviews were mixed and the film flopped at the box office, failing to earn back its production costs. Soon after, Disney fired most of those whose hands had touched it, according to James B. Stewart in his 2005 book "DisneyWar."

As a 25th-anniversary DVD reaches stores this week, "The Black Cauldron" will try to recoup some of its losses. The balance sheet may improve, but not much can be done about the movie itself: It's so-so at best.

The fault doesn't lie with the source material. "The Black Cauldron" was loosely based on "The Chronicles of Prydain," a series of novels written in the 1960s by Lloyd Alexander. In the field of fantasy literature for children, British authors traditionally have dominated: Lewis Carroll, C.S. Lewis, J.K. Rowling and others. Yet Alexander, a Philadelphia native, arguably deserves a place among these superstars. He may be the form's finest American practitioner.

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"He's the best," says Michael O. Tunnell, a professor at Brigham Young University and the author of "The Prydain Companion," a reference guide.

Earlier this year, Prof. Tunnell and his BYU colleague James S. Jacobs oversaw the opening of a campus exhibit on Alexander, who died three years ago. Visitors to the permanent display in the Harold B. Lee Library can see a re-creation of what Alexander called his "box," the household office in which he wrote. It includes everything, from his manual typewriter—he never replaced it with a computer—to the wastepaper basket beneath his desk.

Success didn't come easily for Alexander. Born in 1924, he knew as early as adolescence that he wanted to write. Yet his Depression-era parents discouraged him, warning that a scribbler's life was hard. "It turned out they were quite right, but I didn't believe them at the time," he said in a 1994 video.

During World War II, he served in the Army. His postings included a spell in Wales, an experience that would bear literary fruit later on. He also lived in Paris, where he met the woman who became his wife. Upon his return to Pennsylvania, he struggled just as his parents had predicted. He worked at odd jobs, translated Jean-Paul Sartre into English, and tried to make a name for himself.

Alexander wrote one novel after another but failed to find a publisher. His greatest literary success in those early years may have been a short-story contest for Writer's Digest in which he finished in 29th place.

Yet he kept at it. In time, he became known for his work ethic. Seven days a week, he would begin pecking away at his typewriter by 3:30 a.m. He ate his only meal in the early afternoon. He was a homebody who rarely wandered beyond the local post office and grocery store.

By the early 1960s, Alexander was finally in print and enjoying modest success with his novels. Then he made the decision that would change his life: He wrote a book for kids. "Time Cat," about a talking feline who leads his owner on a time- traveling tour of history, came out in 1963.

This set the stage for "The Book of Three." It was the first of five titles about a boy named Taran, an assistant pig-keeper who finds himself thrust into an epic struggle between good and evil in the magical land of Prydain. The setting was inspired by Wales—"Prydain" is the Welsh word for Britain—and the adventures were drawn partly from "The Mabinogion," a collection of Welsh legends.

The books landed at precisely the right moment. J.R.R. Tolkien was enjoying his first big burst of popularity and Alexander offered a new stomping ground for veterans of Middle Earth. By targeting younger readers, "The Chronicles of Prydain" are closer in spirit to "The Hobbit" than "The Lord of the Rings." They're also funnier. Amid all the adventure, Alexander was careful to include episodes of comic relief.

The fifth and concluding volume, "The High King," arrived in 1968 and won the Newberry Medal, the most prestigious award in American literature for children. A short sixth book, "The Foundling," came out in 1973. Its tales supplement rather than continue the Prydain series, which has never gone out of print.

For Alexander, turning to an audience of boys and girls was liberating. "I found myself able to deal with things that I could never even express writing for adults," he said. "The child's book is as serious an art form as anything else." In his stories, he emphasized the importance of courage and duty, the conundrum of concentrated power, and the notion that where you're going matters more than where you're from.

"Fantasy is hardly an escape from reality," he said of his preferred genre. "It's a way of understanding it."

In all, Alexander wrote some three-dozen novels for children. Wealth and fame seemed not to affect him, except for the fan mail. He made an effort to reply to all of his correspondents.

When he died at the age of 83 in 2007, just a few weeks after his wife, he was still driving the Chevy Nova that he had bought brand new in 1972. In 35 years, according to Profs. Jacobs and Tunnell, the odometer had recorded only a little more than 47,000 miles—or about 1,400 miles per year. Apparently he was too busy escorting millions of readers on journeys through his vivid imagination.

Mr. Miller is the author of "The First Assassin" (AmazonEncore) and blogs at HeyMiller.com.