PORTLAND, Ore.: 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, Ore., after several weeks of health concerns, her son, Theo Downes-Le Guin said Tuesday.

“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’s first novel was Rocannon’s World in 1966 but she gained fame three years later with The Left Hand of Darkness, which won the Hugo and Nebula awards — top science fiction prizes — and conjures a radical change in gender roles well before the rise of the transgender community.

The book imagines a future society in which people are equally male and female and also dramatizes the perils of tyranny, violence and conformity.

Her best-known works, the Earthsea books, have sold in the millions worldwide and have been translated into 16 languages. She also produced volumes of short stories, poetry, essays and literature for young adults.

Le Guin’s work also won the Newbery Medal, the top honor for American children’s literature. Last year, she was inducted into the American Academy of Arts and Letters.