(CNN) -- Author Philip Jose Farmer died in his sleep Wednesday, according to a message on his Web site.
Farmer was known for his science-fiction and fantasy novels and short stories. He was 91.
His most popular work was his "Riverworld" series, written in the 1970s.
Joe Lansdale, a critic, writer and friend of Farmer's, credited Farmer with changing the face of science fiction.
"I just can't begin to tell you how important he is to the field as well as other fields," Lansdale said.
Critics say Farmer, who was born in Indiana but spent the rest of his life in Peoria, Illinois, was the first author to address adult sexual themes in science-fiction novels.
Jonathan Strahan, an editor and critic for Locus magazine, said Farmer treated sex seriously, not in a juvenile manner or for cheap thrills.
"It wasn't pornography and it wasn't just about the sex of it," Strahan said. "It was about the sexuality of people in an interesting and intelligent way."
His influence over the science-fiction community is becoming more apparent over time, critics said.
One of the most well-known science-fiction novels to be influenced by Farmer was "Stranger in a Strange Land" by Robert Heinlein. Strahan said Heinlein was good friends with Farmer.
Farmer was one of a handful of writers to win the lifetime achievement award from the World Fantasy Society and the Grand Master Award presented by the Science Fiction Writers of America.
Winning both the awards is rare, Strahan said, because not many writers wrote as expansively in both fields as Farmer did.
Lansdale said Farmer was fearless when it came to the subject matter for his stories.
Farmer's latest books were published by Subterranean Press. His last novel, "The City Beyond Play," was published in 2007.
A statement on Subterranean's Web site Wednesday noted the death of Farmer, "who we have been lucky to publish, but have been even luckier to be readers of for so many years."
"You can imagine it's been a sad day here -- we're still working through our emotions about it," the statement said. "But we can say that our memories of him are good ones. He was always a joy to work with, and we will dearly miss his intelligence and good nature."
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