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

A family statement posted Tuesday on Le Guin's verified Twitter account says she passed away peacefully on Monday.

In addition to her fantasy and science fiction novels, Le Guin wrote short stories, poetry and literature for young adults.

The family of Ursula K. Le Guin is deeply saddened to announce her peaceful death yesterday afternoon. https://t.co/DC7oy16EWD — Ursula K. Le Guin (@ursulaleguin) January 23, 2018

She gained fame in 1969 with "The Left Hand of Darkness," which involves a radical investigation of gender roles.

Her feminist-themed 1983 "Left-Handed Commencement Address" at Mills College was ranked one of the top 100 speeches of the 20th century by researchers at the University of Wisconsin and Texas A&M University.

The Earthsea books have sold in the millions in 16 languages.