Two new stories by the science fiction great Octavia E Butler have been discovered among her papers and are set for release this summer.

The late and much-celebrated Butler can lay claim to a series of firsts: she was the first science fiction novelist to be awarded the prestigious MacArthur "genius" grant, and is acclaimed, with Samuel R Delany, as one of the first African American science fiction authors to achieve widespread fame. When she died unexpectedly in 2006, at the age of 58, her books – including Kindred and the Xenogenesis trilogy - had sold more than a million copies, and the New York Times praised an author whose "evocative, often troubling, novels explore far-reaching issues of race, sex, power and, ultimately, what it means to be human". In 2010, she was posthumously inducted into the Science Fiction and Fantasy Hall of Fame.

Now her long-term agent Merrilee Heifetz has announced that she has discovered two previously unpublished works by Butler with the help of the archivists at the Huntington Library in San Marino, California, where Butler's papers are housed, and that the stories will be published in June by digital press Open Road Integrated Media.

The first work which was found by Heifetz is a novella, A Necessary Being, which Open Road said "showcases Octavia E Butler's ability to create alien yet fully believable 'others'." It is set in a world where the leadership qualities of a dwindling race called the Hao mean they are valuable enough to be captured and forced to govern, and where the Hao Tahneh is forced to rule as both chief and prisoner for 20 years. "She bears her loneliness privately until the day that a Hao youth is spotted wandering into her territory. As her warriors sharpen their weapons, Tahneh must choose between imprisoning the newcomer – and living the rest of her life alone," said Open Road.

The second story, Childminder, was commissioned by Harlan Ellison – Butler's mentor – for his never-published anthology The Last Dangerous Visions, and sees a telepath attempt to help a young girl harness her growing powers. "But in the richly-evocative fiction of Octavia E Butler, mentorship is a rocky path, and every lesson comes at a price," said Open Road. The pieces will be released together as an ebook, Unexpected Stories.

"I worked with Octavia for many years and saw firsthand the devotion her readers expressed for her and her work. To be able to publish new stories for those readers is a joyful occasion," said Betsy Mitchell at the digital publisher.

Butler's best known novel, Kindred, tells of a young black woman who travels back in time to the deep south of the 19th century, where she must protect her slave-owning ancestor to secure her own existence. "I didn't like seeing her go through back doors," Butler was quoted as saying in the New York Times. "If my mother hadn't put up with all those humiliations, I wouldn't have eaten very well or lived very comfortably. So I wanted to write a novel that would make others feel the history: the pain and fear that black people have had to live through in order to endure."

As a child, said Heifetz, Butler "said she wanted to be a writer, and they said, 'you can't be a writer. Black women aren't writers.'" But the author told the New York Times that she was moved to continue because "when I began writing science fiction, when I began reading, heck, I wasn't in any of this stuff I read. The only black people you found were occasional characters or characters who were so feeble-witted that they couldn't manage anything, anyway. I wrote myself in, since I'm me and I'm here and I'm writing."

Open Road praised her as an author whose fiction "urged readers to look at their world in fresh, unfamiliar ways". In a video, the writer NK Jemisin said "her stories really weren't about aliens or vampires ... they were about people, and they were about all the ways people were alien to themselves. She was using that one step removed to turn a mirror on us, and I'm glad she was willing to hold up that mirror and say 'look at how horrible we are, let's not be that bad'."

Unexpected Stories will be published on 24 June this year.