His imagination is certainly of the first rank - Time

Philip Josť Farmer is legendary for having so many series running at the same time. During the mid seventies, while fans were waiting on the next Riverworld and World of Tiers books, a writer's block stopped him from writing about those worlds. He could only work with new ideas, or with other people's ideas, so he began a few more series; Opar, Paul Eyre, Greatheart Silver and the fictional authors and worked with those until he was able to return to what he fans were calling for.

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The Sturch
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Farmer's first science fiction story, The Lovers, was set in a future where the Church and State had combined into the Sturch. When the novella was expanded in 1961 much of the new content were details about the history of the current religion. The other two stories are not necsassarily sequels to The Lovers but they are set in the same future.
Father Carmody
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One of Farmer's more interesting characters, Father John Carmody has several adventures throughout the 1950's and into the 1960's. The most important story, Night of Light was expanded into a novel in 1966, and the original stories were collected in one volume as FATHER TO THE STARS in 1981.
World of Tiers
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While Riverworld is Farmer's best known series, informal polls done at this web page have shown that the Science Fiction Book Club collections of the World of Tiers novels, have introduced more fans to Farmer than anything else. Considered by some critics to be more adventure tales that science fiction, the first book, THE MAKER OF UNIVERSES, did introduce the idea of "Pocket Universes". Also, Farmer's most loved protagonist Kickaha, romps through most of these books outdoing Tarzan himself for daring feats of skill and strength.
Harold Childe
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In 1968 Farmer added to his reputation for mixing science fiction and sex. Porn publisher Brandon house, started a new line of books to be put out under the Essex House imprint. These books would be written by established authors who could write about anything they wanted, as long as they had sex in them. What Farmer came up with will blow your mind.
Lord Grandrith - Doc Caliban
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Yet another amazing idea from the imagination of Philip Josť Farmer. A FEAST UNKNOWN is his version of pulp heroes, Tarzan and Doc Savage, with sex drives. In this case sex drives gone haywire, but that is part of the plot and I don't want to give it away here. Definitely an "adult" novel but with so many great ideas, one you don't want to miss. The sequels are toned done to mere violence and are not as explicit as the first book.
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Considered by many to be one of the greatest concepts in all of science fiction. Everyone who has ever lived on Earth, from cavemen to 1984, is resurrected along the banks of a million mile long river. Food is provided three times a day through "grails" and if you die you wake up the next morning somewhere else along the river banks. Characters from any point in history meet, interact (and usually go to war) in these great books.
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The idea for this series began in 1971 with the short story, The Sliced-Crosswise Only-On-Tuesday World and was finally expanded to it's full potential as Farmer's last great series in 1985 with the novel DAYWORLD. Earth is so over populated that people are only allow to live one day in seven. The other six days they are put in suspended animation or "stoned". Each day a different person lives in your apartment, a different persion goes to your office and does your job. Unless you are a daybreaker, living seven different lives, one each day of the week.
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12,000 years ago the vast empire of Khokarsa stretches along the shores of ancient Africa's two great inland seas. The hero Hadon is from Opar, the lost city from Edgar Rice Burroughs's Tarzan novels, but reimagined in its glory days against a greater civilization of Farmer's own creation.
Fictional Authors
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This series was begun in the mid 1970's when Farmer had a writer's block that would not allow him to work on the books that his fans were waiting for; the next installments of The World of Tiers and Riverworld series. By writing as Kurt Vonnegut's character, sad sack science fiction author Kilgore Trout instead of himself, the writer's block disappeared, then he just kept on going.
Ralph von Wau Wau
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This series is an offshoot of the Fictional Author series that took on a life of its own when Ralph walked into Callahan's Place.
Paul Eyre
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Written in 1974 and 1975 for CONTINUUM, a series of four books with several continuing stories. This story is about a man and the strange things that happen to him after his encounter with a space ship. The stories were collected and published as STATIONS OF THE NIGHTMARE in 1982.
Greatheart Silver
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These stories were written in the mid 1970s for WEIRD HEROES a series of books that were trying to recapture the feeling of the pulp magazines of the 1930s and 1940s. The final story is one of the funniest Farmer has ever written. The stories were collected and published as GREATHEART SILVER in 1982.