The board game of Azad in Iain M. Banks's The Player of Games is so complex and wide-ranging it resembles life. The entire structure of the interstellar Empire of Azad is informed and held together (and named after) the game, used to settle commercial, military, societal and other disputes. The winner of the great tournament is made Emperor. Playing Azad is very Serious Business.
This is a reference to The Glass Bead Game by Hermann Hesse, in which the game really does exactly mirror life itself.
The Glass Bead Game by Hermann Hesse features the eponymous game. There are universities devoted to it which appear to be the only way to get a tertiary education and the study of the game takes over people's lives as if they were joining a religious order.
Since the Glass Bead Game (which isn't really a game) is essentially the synthesis of all art and knowledge, and has replaced art, it actually is serious business.
In one of Rex Stout's Nero Wolfe novels, Archie Goodwin comes downstairs to find an enraged Wolfe burning a dictionary in the fireplace. The problem? The dictionary gave "imply" as a synonym of "infer".
Actually, that was only one of many horrors in that (real) dictionary.
In Gullivers Travels, the nations of Lilliput and Blefiscu are engaged in a war over which end to open a hard-boiled egg, the wide end or the narrow end. Swift intended this as a not-so-subtle satire of both the schism between Catholic and Protestant Christians and the rivalry between England and France.
American Psycho does this with almost everything relating to appearances — the book commonly spends about half a page of every chapter just listing the clothing, perfume and other brands of status symbols (such as watches) Bateman and his colleagues are wearing at that moment. If you actually know the individual articles of clothing being described, you'd know that they'd look ridiculous together.
Conor Kostick's novel EPIC is a kind of satire of this tendency. It focuses on a group of far-future colonists using a sword and sorcery MMORPG as a system of government. Also, their economy is based around it too - a player's in-game money is their real world money, thus leading to players spending most of their free time grinding lowest level monsters (since they dare not risk invoking the games permanent character death system). Not only is their world slowly stagnating, but the Serious Business manner everyone plays the game in is poisoning it to the point that the games AI wants to be put down. By the end of the story, this results in the destruction of the game world, all its beauty and possibility wasted - an implicit end result of uncontrolled Serious Business.
The Thursday Next novel series is set in an alternate-universe England. One of the key elements of this universe is that a countless number of things are Serious Business; for instance, people who disagree with the surrealist movement organize riots, and the titular character is in a government-sponsored division of law enforcement devoted solely to literary theft. There are also door-to-door evangelists who go around trying to convince people of who really wrote Shakespeare's plays.
Dr. Seuss's Butter Battle Book is a clear parody of the Cold War and accompanying Soviet/U.S. arms race. The issue that caused the division and started the whole thing off? Which side of the bread is buttered.
In another (ETA: "The Zax"), two creatures refuse to step even a single inch to the side to let the other move on; the creatures remain standing, toe-to-toe, for the rest of eternity.
Finally, as a swipe at prejudice, the Sneetches spend all their money adding and removing stars from their bellies so they won't look like the other side. This is a satirization of how racism is idiocy, so maybe it really is Serious Business after all.
In Snow Crash, the whole first section is mock-heroic and elevates pizza delivery as serious business. Electronic timers are placed on each pizza box from the very second the order is placed, and should the thirty minute timer expire then... well, what happens next begins with the owner of the pizza company personally visiting the wronged customer and apologising profusely. Since that boss is the Don, each and every pizza delivery driver knows well enough that they'd be better off breaking the speed limit, their cars, the sound barrier, anything, than deliver a pizza at 30:01 or later.
Stephenson seems to be somewhat fond of turning mundane everyday situations into Serious Business: consider Randy tackling his everyday bowl of Cap'n Crunch in Cryptonomicon, and that's just the tip of the iceberg.
Also the multiple-page memo in Snow Crash dealing with intra-office purchase and use of toilet paper. Sadly, Truth in Television for anyone who's ever worked in a stuffy bureaucratic situation.
In the novel The Kite Runner kite fighting is portrayed a little like this, except not all year long. Apparently true.
Parodied in Interesting Times, when Cohen realizes that the Agatean Empire's obsession with tradition (such as the tea ceremony) is part of the reason it has stagnated. He starts the winds of change by telling his new Grand Vizier (a man unsuited to the job, who will therefore be much better at it than the previous incumbent) that there's a new tea ceremony, and it doesn't take three hours, because it goes like this: "Tea up, luv. Milk? Sugar? Scone? You want another?"
Going Postal features Stanley Howler, who takes pins Very Seriously Indeed, and is regarded as "a bit weird about pins" even by other pin collectors. When stamps are invented, he gets over it and goes crazy about stamps instead. He is promptly appointed to be in charge of the stamp department of the Post Office.
Unseen Academicals tackles (no pun intended) this with regards to association football. Justified in that a bequest to Unseen University means that not playing football will lose the wizards most of their food money. (And for the wizards, food has always been serious business.)
It becomes even more justified when Lord Ventinari decides to use the football match to break the power of the football gangs that are becoming a major disruptive influence in the city. While it is Serious Business to the common hooligans, to the "faces" of the Shove this is a direct attack on their political power base.
Miscellaneous: The Fool's Guild. They treat comedy as deadly serious; they won't tell a joke until it's spent years being dissected in review by the guild masters, funerals are conducted with the whole custard-down-the-pants business treated with the same grave solemnity of a eulogy, and they have weaponized slapstick gags for serious combat. On top of that, any character who is a jester, clown, fool, or other idiotic-looking joculator, automatically gets the trope Hilariously Abusive Childhood applied to him. Yes, even if he's just an incidental character whose past never comes up.
They're so indoctrinated that they consider their greasepaint makeup to be their true face, and said faces can be inherited down the family line. Copies of the faces are made on eggshells to ensure no one accidentally uses a face assigned to someone else. Using someone else's face, especially on purpose, is viewed as being roughly equal to murder.
Catherine Asaro's Skolian Saga novels The Last Hawk and A Roll of the Dice describe a world run by a game of dice called Quis. In The Ruby Dice, the game gets bigger...
The Rape of the Lock is a merciless mocking of what was, at the time, real life Serious Business. In essence: Some guy cuts a lock of hair from a woman's head, causing much social drama.
The Way Series by Greg Bear: Eon and Eternity revolve around an incredibly high-tech civilization which arose from the ashes of a late 20th century nuclear war. A large chunk of the population eschew that technology, are leery of advances which they see as dehumanising, and strive to live a "primitive" lifestyle based on the technology and norms of late 20th and early 21st century Earth. And their entire philosophy and religion is based on the teachings of... Ralph Nader's consumer advocacy in the 70s and 80s.
They're even called "Naderites." The characters from the past lampshade this by commenting "Anyone tell him yet?"
The 39 Clues are Serious Business for all four branches of the Cahill family.
Although seeing as the prize for finding all 39 is super strength, intelligence, creativity and strategy skills that could grant you world domination... well, the way all the characters take it seriously doesn't seem too farfetched.
Quidditch in the Harry Potter series is treated as such, especially the matches between Hogwarts houses.
Partly lampshaded by Harry when he says Hermione will never understand what serious business Quidditch is.
Justified though, if you take into account that Quidditch is the wizarding sport. In the real world, there's a crazy amount of different sports and an even crazier amount of fans of every sport. Imagine if there was in fact only one single sport in existence, and all those fans were fans of the same sport. The world wouldn't be divided into groups of people liking different sports, it would be divided into two groups only: fans, and people who didn't care. That's how big Quidditch is.
On the other hand while it is the wizarding sport, how overly serious it is treated is debatable. Many times it is just treated like a sport, and Hermione simply doesn't understand sports.
Have you ever opened Quidditch Through the Ages? America has their own sport, Quodpot. Quidditch is just THE British/Scottish Sport, and it is played worldwide and is held in great honor. Also, Quidditch is taken to a higher extreme with Japan's Toyohashi Tengu, they burn their brooms if they lose a match.
It's a reflection of the general British obsession with football (soccer). There are other sports but none of them seem to generate quite the same level of drunken hooliganism, violence, rioting and obsessive press coverage/water cooler discussion of things like player transfers or somebody's striker getting injured. Quidditch is just an exaggeration of that - the wizarding world goes even crazier about it because their next alternative seems to be Gobstones or Exploding Snap, neither of which are quite as exciting.
In Muletrain to Maggody, some of the Civil War reenactors are so intensely devoted to their hobby lifestyleobsession that they deliberately collect welts and blisters from overly-stiff boots, drink contaminated creek water to contract historically-accurate diarrhea, and incur 2nd degree sunburns while stubbornly playing dead, all in the name of "not being a farb". One would've sworn off dentistry for that just-two-teeth-left-and-they're-blackened-stumps look, had his wife not threatened divorce.
In the Blandings Castle novels, the Shropshire fat pig contest is Serious Business indeed. So much so that the quest for the first prize for the Empress of Blandings drives the plot of several novels.
Anything Bertie Wooster or any of his pals attempt. Bertie compares his attempt to play a prank on Tuppy Glossop to the crusades (to name just one example).
For Jeeves clothing is very Serious Business indeed, he is willing to destroy things of Bertie's that he doesn't like and bad fashion sometimes makes him physically ill.
Golf in the Oldest Member stories is treated like a religion, presided over by the Oldest Member.
Cricket in the early portions of the Psmith series. (School sports in Boarding School stories were always serious business.)
An apparently nonhumorous one is found in the Takeshi Kovacs book Woken Furies, in which the grizzled mercenary recalls that the warring youth gangs of Harlan's World are divided into surfers and scuba divers. Kovacs himself joined a crew of bad ass scuba divers whose motto was "Dive deep. Die free. Leave the scum on the surface." One can only imagine hardened street thugs getting passionate about drifting around underwater looking at fish.
In The Count of Monte Cristo, potential newspaper errors are very serious business. Two people nearly died because one believed that a story was incorrect. However, this is Truth in Television, as people really did take public opinion that seriously.
The Phantom Tollbooth has everyone in the Kingdom of Knowledge being incredibly defensive and vocal about what bit of knowledge they specialize in. The entire kingdom was split between two brothers, one who thought words and letters were superior to numbers, and the other who thought vice versa. Summed up by the main character saying, "Everyone's so terribly sensitive about what they know best".
Don Quixote: Serious Business is one of the principal themes of the novel: The first part, only Don Quixote is affected with the chivalry lifestyle, but in the second part, a sizable portion of Spain's population takes it far more seriously than it should be. There are various examples:
Don Quixote: The very last stage of Alonso Quixano obsession with chivalry books and the first stage of his true madness (and also to show exactly how out of touch with reality he really is: Part I, Chapter I shows us how important are the chivalry books for him: (Ganelon was the guy who betrayed Roland at Roncesvalles and who becomes, with Mordred and Judas, one of the great exemplars of treachery for the mediŠval period).
"To have a bout of kicking at that traitor of a Ganelon he would have given his housekeeper, and his niece into the bargain."
The Duke and the Duchess: They spent a lot of money and organize truly Massive Multiplayer Scam (Dulcineaĺs enchantment has all the people in their castle, the Insula Barataria involucres all the people of a town) only to laugh at Don Quixote and Sancho.
Older Than Feudalism: The sphinx that troubled Thebes in the story of Oedipus treated her riddles like Serious Business. When Oedipus figured out the answer, she actually committed suicide. Even before that, she'd only eat people once they'd actually made an attempt to answer the riddle, and gotten it wrong.
Dave Barry in Cyberspace has a legal contract for "unconditionally and irrevocably" giving someone else a stick of chewing gum.
In Mervyn Peake's Gormenghast (and its BBC mini-series adaptation), the Rituals take up the Earl of Groan's days, but the meanings of the gestures — things like scratching a mark of the proper length on the back of a door on a specific metal cabinet with a knife of the proper mark — have been lost in the dust of the castle's long history. Doesn't stop the Earl's secretary from being a stickler for doing things just so, and eventually drives the seventy-seventh earl, a mere child when he comes into his title, to make numerous escape attempts.