Isaac Asimov's Black Widower mystery stories have the out of print books in "Sunset on the Water", a lucky coin in "The Lucky Piece", and the data in "The Alibi". The data is a somewhat lampshaded MacGuffin, as the government agent telling the story points out that the details are unimportant, but still secret.
The three Silmarils in The Silmarillion act as MacGuffins for most of the plot. The Silmarils are not precisely MacGuffins, because they demonstrate a few powers (such as piercing the barriers around Doriath and Valinor), but the characters desire the Silmarils only for their beauty. Wars ensue as many characters fight for possession of the Silmarils, never intending to invoke their powers.
The dwarves reclaiming their ancestral home is the MacGuffin in the first two acts. The Arkenstone doesn't become significant until the third act.
The One Ring from The Lord of the Rings (though commonly cited as an example) is explicitly NOT a MacGuffin, as its power to corrupt anyone who comes near it is a major driver of the plot, and it is arguably an independent character in its own right. For one thing, it got Boromir killed, and would have been impossible to destroy were it not for Gollum's intervention.
The key reason being that unlike normal MacGuffins you cannot replace it with another MacGuffin and have the effect be the same. Even the aforementioned Silmarils doesn't seem to possess a will of their own, as the One Ring is actually the core essence of an intermediate deity of the Earth. Or a powerful Maiar of Aulė, if you prefer.
However, the plot to destroy the One Ring is a MacGuffin.
Lampshaded in the David Bischoff novel Star Fall, in which the protagonist transfers bodies for a vacation. Unwittingly, he ends up with an illegally modified artificial body capable of all sorts of sci-fi/007 skullduggery, which any number of elements are after. The type of illegal artificial body he is inhabiting is called, you guessed it, a MacGuffin.
Fyodor Pavlovich Karamazov's vast fortune in The Brothers Karamazov is said to exist, but even the narrator casts aspersions as to how much money he really has, if any. The sons' owed inheritance is the MacGuffin which gets the plot moving in the beginning, but it is only brought up past the middle of the book in passing. The argument could also be made that the sub-plot involving the schoolboys, which is almost entirely unrelated to the main events of the novel, is a MacGuffin to explore some other themes of spirituality.
Lampshaded in Walking on Glass by Iain Banks. At the end of Steven's story, Steven finds a box of McGuffin's Zen Brand matches, on the back of which is written the answer to Quiss and Ajayi's riddle. Quiss and Ajayi have forfeited all future attempts to answer the riddle, because Quiss has destroyed the Game Table, but we know that their current attempt, earned by completing a game of "Tunnel", will be correct because Ajayi finds a copy of Walking on Glass in the remains of the Game Table.
The plot of the classic satirical novel The Twelve Chairs by Ilf and Petrov revolves around a treasure hidden in a chair. By the time the main characters find it, the treasure is long gone
One of Simon Hawke's Time Wars books plays out around the plot of The Three Musketeers, and the villain replaces the diamond studs with future-tech explosives, planning to detonate them in the Queen's court and thrown history off track - thus making them a McGuffin for a different reason.
In some of the Jeeves and Wooster stories of P. G. Wodehouse, a silver tea-set creamer, hideously forged in the shape of a cow, becomes the focus of a on-going multi-cornered power-struggle. In other stories, the French chef Anatole is deployed as a Living MacGuffin.
Lampshaded in Italian writer collective Wu Ming's novel Fifty Four, which features a very important TV set manufactured by McGuffin Electrics.
The enormous, apocalyptic disaster at the beginning of The Road. We never find out what it was, because that's not important.
Eric Flint made up a MacGuffin for his book 1632 (he originally thought he was only going to write one book, not a series) called an Assiti Shard that transports a spherical area through time and space to an Alternate Universe. Flint openly states that he made the things up so that he'd have an easy way to create various Alternate History and Science Fiction books.
The entire universe of intelligent life revolves around a MacGuffin in William Sleator's Interstellar Pig, and then inverts it. First, it's about an object you need to be holding by the end of a boardgame to win it, with the MacGuffin in question called the Piggy. Then, it turns out the Piggy is for real, and everyone is trying to get ahold of it so their species won't be vaporized by the end of the game. Once they've held it for a while, though, the Piggy tells them that if they don't pawn it off on some other species soon, the Piggy will blow a hole in the universe and kill whoever has it. The trope is folded back on itself again, as the human in the game gives the Piggy to some carnivorous fungi, and watches the departing spaceship leaving Earth carefully to see if it will make awesome fireworks.It doesn't.
In the Shadowleague books, there's the Heart of Myrial, and the Hierarch's ring is a borderline example after Gilarra loses it to the Ak'Zahar.
The King's Ruby in Finn Family Moomintroll. The Groke is after Thingumy and Bob because they stole it from her, and the Magician (translated as "the Hobgoblin") has been searching the whole solar system for it all his life. It doesn't actually do anything aside from looking almost supernaturally nice.
In Mikhail Akhmanov's Earth Shadow, Dick Simon is sent by the civilized worlds to find out the fate of Earth That Was, which was cut off from the Portal Network at the end of the Exodus. He spends most of the novel looking for the Poltava, a top-of-the-line naval triplehulled cruiser, built shortly before the Exodus. He needs the ship's missiles to destroy a Lunar base that is the cause of the portal interference. He finally finds the derelict ship in a grotto under a mountain. Unfortunately, the missiles have all been used up. He ends up using a completely different (and easier) method of shutting down the transmitter. Had be done that from the start, the book would've been only ten pages long.
The Saghred in Lisa Shearin's fantasy series: an evil stone of cataclysmic power accidentally bonded to the main character. Everyone is after it, but Raine just wants to get rid of it. Also an Artifact of Doom and a Clingy MacGuffin.
In The Scar, the magusfin is this for the pursuing grindylow... or so Bellis thinks.
The painting "Moscow Asylum" in David Madsen's USSA, for a while. The protagonist wants to find because it's valuable to the artist, and he wants some information from the artist. A bunch of goons who steal it from him only do so because they assume it has some more intrinsic value to case he working on.
Robert A. Heinlein's Glory Road contains a classic example: the Egg of the Phoenix, which must be recovered by the hero Oscar after having been stolen from the Empress of Fifty Universes. It has a function, but that function is irrelevant to the rescue plot, and only becomes important in the third part of the story by virtue of its Deconstruction of the Standard Hero Reward.
The Ghost Fleet in Startide Rising plays no larger role than to sic the whole galaxy on one damaged spaceship full of dolphins. Everyone, and I mean everyone wants to know where it is.
In Michael Flynn's The January Dancer, the harper claims that the object of The Quest is not important; what mattered was Jason and Medea, not the Golden Fleece. The scarred man objects: had he gone after the Tin Whistle or the Aluminum Coffeepot, the failure would have been different.
We'd stolen wills for ballast and a crew of missing heirs.
In Poul Anderson's "Time Patrol", it's the chest of radioactive materials found in a Dark Ages tomb during Victorian Britain. They don't merely have to get it back, they have to get it back before it was buried.
In the historical novel Wings of Dawn: The cache of Druidic lore and treasure that can be used to prove the Ancient Conspiracy's existence. Also the cache of books from the Orient that Thomas has, which both conspiracies are trying to get their hands on.
David Eddings intentionally included these in his books (since the books themselves were built on the premise of taking overused tropes and cliches and making them work), mainly the Orb of Aldur and Bhelliom, although both break the rules by being both plot-relevant and useful after they're claimed by the protagonists. The Sardion is a more exact case: both the heroes and the villains need to reach it for the final confrontation, but even though it is supposed to be a counterpart to the Orb, it never does anything special (except occasionally charm people into moving it to another location, and that is never explicitly shown), and it's destroyed in the final confrontation.
In the web-novel Domina, the toy box is one. It's a more advanced form of the already-powerful toy maker, allowing for easier and faster Bio-Augmentation. The aves (bird people) stole it to get wings, and quite a few other cultures want it for their own purposes.
Played with in A.L. Phillips's The Quest of the Unaligned. While the Prince's Crown that the titular Quest focuses on actually behaves as more of a Plot Device, since its unique magical properties are responsible for the entire last third of the plot, almost nobody knows about its powers. Therefore, for most of the first half of the book, the Crown is treated as a MacGuffin, even though it really isn't.
In Stephanie Burgis's A Most Improper Magick, the missing will, which Sir Neville carries about with him. When Mr. Collingwood guesses he's tortured with guilt for hiding it and claiming all their mother's estate, Kat points out that doesn't sound like Sir Neville. it was actually to keep it hidden until he could find a way to break the magic preserving it.
The High Window has Marlowe tracking down the Brasher Dubloon, a legendary coin worth a fortune that leaves a trail of dead thieves behind it; come the ending, it turns out a minor character sold it for a new start with a clean slate, but it's unimportant considering Marlowe uncovers a framing and a few murders in the process.
The Long Goodbye has Marlowe's drinking buddy, Terry Lennox, fleeing the country and paying Marlowe with a $5000 bill. Marlowe, believing he hasn't earned the sum of cash, spends the entire plot refusing to spend it. Its only significant uses are: to involve Marlowe in the second case; and so Marlowe can pay it back to Lennox, giving them an excuse to meet up again in the conclusion.
In Kazuo Ishiguro's Never Let Me Go, the tape of Judy Bridgewater's Songs After Dark is one, but lampshaded to the point of disrupting the plot: "I stood there quite still, looking at the plastic case, unsure whether or not I was delighted. For a second, it even felt like a mistake. The tape had been the perfect excuse for all this fun, and now it turned up, we'd have to stop."