Ellery Queen was the pen name for two cousins, Frederic Dannay and Manfred Bennington Lee, who wrote novels and short stories about a fictional detective character named Ellery Queen.
 At various points in their history, the cousins allowed the name of Ellery Queen to be used as a house name; novels written by other writers were published under that name.
 The cousins also wrote four novels that were published under the name "Barnaby Ross" and they later allowed that name to be used as a house name.
 Several of the main series novels were also ghost-written, but those were controlled and edited by the cousins.
Showboat World (original title: The Magnificent Showboats of the Lower Vissel River, Lune XXIII, Big Planet), written in 1975, is the second, stand-alone novel in a pair of science fiction novels by Jack Vance (the first being "Big Planet") that share the same setting, a backward, lawless, metal-poor world called Big Planet.
 The plot structure which involves a series of dramatic presentations, often with humorous consequences, has parallels with Vance's 1965 novel Space Opera.
The Magazine of Fantasy & Science Fiction (usually referred to as F&SF) is a U.S. fantasy and science fiction magazine first published in 1949 by Fantasy House, a subsidiary of Lawrence Spivak's Mercury Press.
 Editors Anthony Boucher and J. Francis McComas had approached Spivak in the mid-1940s about creating a fantasy companion to Spivak's existing mystery title, "Ellery Queen's Mystery Magazine".
 The first issue was titled "The Magazine of Fantasy", but the decision was quickly made to include science fiction as well as fantasy, and the title was changed correspondingly with the second issue.
 "F&SF" was quite different in presentation from the existing science fiction magazines of the day, most of which were in pulp format: it had no interior illustrations, no letter column, and text in a single column format, which in the opinion of science fiction historian Mike Ashley "set "F&SF" apart, giving it the air and authority of a superior magazine".
Stephen Marlowe (born Milton Lesser, (1928--) 7, 1928 in Brooklyn, New York, died 22, 2008(2008--) (aged 79 ) , in Williamsburg, Virginia) was an American author of science fiction, mystery novels, and fictional autobiographies of Christopher Columbus, Miguel de Cervantes, and Edgar Allan Poe.
 He is best known for his detective character Chester Drum, whom he created in the 1955 novel "The Second Longest Night".
 Lesser also wrote under the pseudonyms Adam Chase, Andrew Frazer, C.H. Thames, Jason Ridgway, Stephen Wilder and Ellery Queen.
Big Planet is the first of two stand-alone science fiction novels by Jack Vance (the other being "Showboat World") which share the same setting: an immense, but metal-poor and backward world called Big Planet.
John Holbrook "Jack" Vance (August 28, 1916 – May 26, 2013) was an American mystery, fantasy, and science fiction writer.
 Though most of his work has been published under the name Jack Vance, he also wrote 9 mystery novels using his full name John Holbrook Vance, three under the pseudonym Ellery Queen, and one each using the pseudonyms Alan Wade, Peter Held, John van See, and Jay Kavanse.
 Some editions of his published works give his year of birth as 1920.
Ellery Queen's Mystery Magazine is an American digest size fiction magazine specializing in crime fiction, particularly detective fiction, and mystery fiction.
 Launched in fall 1941 by Mercury Press, EQMM is named after the fictitious author Ellery Queen, who wrote novels and short stories about a fictional detective named Ellery Queen.
Maske: Thaery is a science fiction novel by Jack Vance set in his Gaean Reach milieu, which was first published as a paperback by Berkley Books, the science-fiction imprint of Putnam, in 1976.
 "Maske: Thaery" marks the beginning of the period when Vance's novels were published exclusively straight to paperback, whereas prior to this the majority had first appeared in science fiction magazines, the last such examples being "Durdane" trilogy, serialised in "The Magazine of Fantasy & Science Fiction" from 1971 to 1973, "The Gray Prince", serialised in "Amazing Science Fiction" in 1974, and "", also serialised in "Amazing Science Fiction" in 1974.
 The twofold title of "Maske: Thaery", with its separating colon, would suggest that it was originally intended to be part of a serial, similar to the Alastor novels ("", "", and ""), although no further volumes were written: instead, Vance completed his Demon Princes series, after a hiatus of more than a decade, with "The Face" (1979) and "The Book of Dreams" (1981), before embarking on his Lyonesse trilogy (1983–89).
Avram Davidson (April 23, 1923 – May 8, 1993) was an American writer of fantasy fiction, science fiction, and crime fiction, as well as the author of many stories that do not fit into a genre niche.
 He won a Hugo Award and three World Fantasy Awards in the science fiction and fantasy genre, a World Fantasy Life Achievement award, and an "Ellery Queen's Mystery Magazine" short story award and an Edgar Award in the mystery genre.
 Davidson edited "The Magazine of Fantasy and Science Fiction" from 1962 to 1964.
 His last novel "" was completed by Grania Davis and was a Nebula Award finalist in 1998. "
The Encyclopedia of Science Fiction" says "he is perhaps sf's most explicitly literary author".
The President of the United States, Detective is a science fiction/mystery short story by H. F. Heard.
 It was originally published in "Ellery Queen's Mystery Magazine" in March 1947, and subsequently republished in "Ellery Queen's Mystery Magazine" in April 1969 and February 1991, in the 1949 anthology "The Queen's Awards", and in the 1975 anthology "Ellery Queen's The Golden 13"; as well, an extended version, named "The Thaw Plan", was published in Heard's 1948 collection "The Lost Cavern and Other Stories of the Fantastic".

Question: Showboat World, written in 1975, is the second, stand-alone novel in a pair of science fiction novels by Jack Vance, was an American mystery, fantasy, and science fiction writer, he also wrote how many mystery novels, under the pseudonym Ellery Queen?
Answer: three