Ursula K. Le Guin, the award-winning science fiction and fantasy writer who explored feminist themes and was best known for her Earthsea books, has died at 88.

Le Guin died suddenly and peacefully Monday at her home in Portland, Oregon, after several weeks of health concerns, her son, Theo Downes-Le Guin said Tuesday.

Le Guin was considered an icon of science fiction and fantasy writing. She published more than 20 novels, a dozen poetry collections, seven essay collections and 13 children's books. Her first novel, Rocannon's World, was released in 1966. Two of her best known works followed shortly after: she published A Wizard of Earthsea in 1968 and The Left Hand of Darkness in 1969. Her Earthsea trilogy has since been compared to J.R.R. Tolkien and C.S. Lewis and sold millions of copies worldwide.

"She left an extraordinary legacy as an artist and as an advocate of peace and critical thinking and fairness, and she was a great mother and wife as well," he said.

"Godspeed into the galaxy," Stephen King tweeted, saying Le Guin was a literary icon, not just a science fiction writer.

Le Guin won an honorary National Book Award in 2014 and warned in her acceptance speech against letting profit define what is considered good literature.

Despite being a finalist for the Pulitzer Prize in 1997 — a rare achievement for a science fiction-fantasy writer — she often criticized the "commercial machinery of bestsellerdom and prizedom."

"I really don't want to watch American literature get sold down the river," Le Guin said in the speech. "We who live by writing and publishing want — and should demand — our fair share of the proceeds. But the name of our beautiful reward is not profit. Its name is freedom."

Le Guin was born in Berkeley, Calif., on Oct. 21, 1929. When she was 11, she had her first offering rejected by the pioneering science fiction magazine, Amazing Stories.

Her themes ranged from children's literature to explorations of Taoism, feminism, anarchy, psychology and sociology to tales of a society where reading and writing are punishable by death and of a scientist who battles aliens to save the world.

Le Guin's many accolades included multiple Hugo and Nebula awards for science fiction, the Newbery Medal for children's literature, the PEN/Malamud Award for short fiction and the National Book Foundation's Medal for Distinguished Contribution to American Letters. In 2000, the U.S. Library of Congress named her a Living Legend for her contribution to American literature.

Neil Gaiman, a fellow Newbery, Hugo and Nebula recipient, mourned her death on Twitter and called Le Guin "the deepest and smartest of the writers."

"Her words are always with us. Some of them are written on my soul," he wrote.

She married Charles Le Guin in Paris in 1953. They moved to Portland and had three children.

Her last book was the essay collection No Time to Spare: Thinking About What Matters, which came out in 2017.

In 1993, Le Guin spoke to Eleanor Wachtel for an episode of Writers & Company.

"You're supposed to start out writing autobiographical novels and stuff, but for me that's the really hard and scary stuff — is getting anything personal in," Le Guin told Wachtel.

You can listen to their entire conversation below.