Larry Niven, after writing his famous science fiction novel Ringworld, later realized that he had ended up writing The Wonderful Wizard of OzIN SPACE!, with the plot and characters mirroring those in the story.
Science-fiction grandmaster Isaac Asimov was open about the fact that his classic Foundation trilogy was The Decline and Fall of the Roman EmpireIN SPACE! This is heavily lampshaded in its Serial Numbers Filed Off sequel by Donald Kingsbury, Psychohistorical Crisis. The protagonist finds a copy of The Decline and Fall of the Roman Empire in an antique shop; he's excited to discover a genuine Earth document, while his mentor glances through it and declares it a clumsy fake that's based on the much more recent history covered by Asimov's novels.
Incidentally, The Trigan Empire is The Decline and Fall of the Roman EmpireON ANOTHER PLANET!
Heinlein's Job: A Comedy of Justice is admittedly James Branch Cabell's Jurgen: A Comedy of JusticeIN CHRISTIANITY!!!
Heck, a substantial fraction of Heinlein's output follows this trope:
"Citizen of the Galaxy" is Rudyard Kipling's "Kim" IN SPACE!
"The Moon is a Harsh Mistress" is the American Revolution IN SPACE!
A bizarre hybrid of the American and Russian revolutions.
Also "Red Planet". Also "Between Planets". The American Revolution was a big thing with Heinlein.
"Rocket Ship Galileo" is Nazis IN SPACE!
"Starman Jones" is Horatio Alger IN SPACE!
Etc. You get the picture. To his credit, Heinlein never pretended to make stuff up out of whole cloth, as he had several characters in several books explain at length. (e.g., Hazel Meade Stone explaining that she was going to do an episode of her radio serial as Shakespeare IN SPACE.)
H.G. Wells's War of the Worlds is colonialism WITH ALIENS! Wells was an outspoken critic of colonialism and War of the Worlds was his way of showing a western audience what colonialism was like from the point of view of the natives. Since European countries often tried to justify colonialism by portraying themselves are more advanced and civilised than those they conquered, he took this to its logical extension by showing England being invaded by an even more advanced civilisation.
Which means that Suzumiya Haruhi is StargirlWITH THE SUPERNATURAL!
Pat Murphy's There and Back Again is, unabashedly and transparently, The HobbitIN SPACE! Space Pirates, good and bad tend to fill in many of the groups in the novels (i.e. the Bilbo equivalent journeys with an all-female group of adventurers, and standing in for Orcs are the Resurrectionists, a Reaver-like bunch who capture people and harvest their organs.
Fred Saberhagen's novel Berserker Fury is the Battle of MidwayIN SPACE! The "island" planet was called 50/50 (halfway or "midway" between two points), spaceships involved were named after the U.S. ships (Stinger for USS Hornet, Venture for USS Enterprise, etc.), and the battle used almost the exact same tactics, among other similarities.
It seems fit to mention that in The Silver Chair, the Emerald Witch enthralls the protagonists and when they try to remember the outside world, insists that their sun is just a lamp ONLY BIGGER AND BETTER, and that their lion is just a cat ONLY BIGGER AND BETTER.
Which is C. S. Lewis skewering the idea that because we can only describe Heaven in terms that amount to "like Earth ONLY BETTER", it doesn't exist.
Stephen Lawhead's King Raven trilogy is Robin HoodIN WALES. (No, really).
The Legend of Bagger Vance is the Bhagavad Gita IN THE SOUTH! ON A GOLF COURSE!
The Story of Edgar Sawtelle is HamletON A FARM IN RURAL WISCONSIN! WITH DOGS!
Wing Commander: the "Expanded Universe" novels written solely by William Forstchen (a historian by training) use this trope quite readily.
End Run is basically Wing Commander's 1942 Doolittle raid on Tokyo, though the attack on the Kilrathi homeworld holds a much more strategically significant role in the novel.
Fleet Action is pretty much Wing Commander's Battle of Midway.
Action Stations is an almost painfully obvious reflection the attack on Pearl Harbor.
John Ringo's Prince Roger series is, at least for the part on Marduk, Xenophon's AnabasisON AN ALIEN PLANET! His ''The Last Centurion is the same... NEXT SUNDAY AD!
