Something Wicked This Way Comes
Ray Bradbury's Something Wicked This Way Comes is another excellent story of an amusement-park-with-creepy-undertones.
If this seems too little to relate it to Westworld, skip directly to Jurassic Park.
Like Westworld, Jurassic Park is a Michael Crichton brainchild that focuses on a theme park collapsing under the weight of its own ambition. Unlike the movie, the book offers the reader something solid to sink their teeth into, with Ian Malcom's fractals and chaos theory underpinning the calamity.
Down and Out in the Magic Kingdom
Cory Doctorow's Down and Out in the Magic Kingdom is set in a future version of Disneyland, in a world beyond currency, where humans are effectively immortal.
It poses many similar issues about the self that Westworld does, though from a very different angle... though in this case those questions revolve around cloning rather than androids.
Sailing to Byzantium
Like Down and Out in the Magic Kingdom, Sailing to Byzantium takes place in a future in which humans have effectively solved the problem of death. Society is largely dedicated to the reconstruction of past civilisations, offering the opportunity to live as the Romans or the Carthaginians did.
The Illustrated Man
As in Westworld, those cities are populated by people who are seen as less than human, the "temporaries."
The Illustrated Man includes a short story called "The Veldt," which features a virtual reality nursery in which children are effectively raised by an increasingly creepy artificial intelligence.
It's a genuinely odd vibe, but one that resonates weirdly well with Westworld's reality-within-a-broader-science-fiction-context.
Larry Niven's Dream Park is a bit of an odd one... it's difficult to call this a "recommendation" because it's not Niven's best work, but it's definitely worth taking a look at Dream Park to see what Westworld might have been like in a world without videogames.
Store Of The Worlds
An NYRB Classics Original
Robert Sheckley was an eccentric master of the American short story, and his tales, whether set in dystopic cityscapes, ultramodern advertising agencies, or aboard spaceships lighting out for hostile planets, are among the most startlingly original of the twentieth century. Today, as the new worlds, alternate universes, and synthetic pleasures Sheckley foretold become our reality, his vision begins to look less absurdist and more prophetic. This retrospective selection, chosen by Jonathan Lethem and Alex Abramovich, brings together the best of Sheckley’s deadpan farces, proving once again that he belongs beside such mordant critics of contemporary mores as Bruce Jay Friedman, Terry Southern, and Thomas Pynchon.
Battle Royale: Remastered
It's sad to live in a world where Battle Royale is easiest to describe in terms of how much it is like The Hunger Games...
It's relevant to Westworld not only for its portrayal of humanity in a situation in which violence is often the first response, but also for Kazuo Kiriyama, who acts as a reflection of Westworld's "man in black."
Do Androids Dream Of Electric Sheep?
In any discussion of androids, robotics, and humanity, Philip K. Dick's Do Androids Dream of Electric Sheep should get a deserved shoutout. You could be forgiven for thinking watching Blade Runner was enough, but there's plenty more here to justify going back.
The Complete Robot
Asimov is rightly the writer we think of first when someone mentions robots. This recommendation could just have easily been made with I, Robot, Robot Dreams, or Robot Visions, but The Complete Robot puts together a fantastic series of short stories that grapple with Asimov's visions of what the future could mean for robotics.