In "The Most Dangerous Game", the climactic, much-anticipated sword fight is completely skipped over. It's only mildly implied afterwards that the protagonist won.
Harry Potter mostly shows only the POV of the title character, which is no problem in the earlier books; in the later books, however, there is a war being fought largely out of view, with the other characters doing various things to fight it that the audience doesn't get the chance to see.
The Final Battle is probably the most jarring example, as there was a lot of action going on with many characters involved. Even the part where Ron and Hermione break into the Chamber of Secrets and destroy a Horcrux with a Basilisk fang happens offscreen. In the other books, the characters involved in the action were usually the trio if at all, or an extended group which Harry was a part of.
This was a deliberate decision on J. K. Rowling's part - in real life, battles are messy, unpredictable affairs, and the "good bits" don't wait until plot-relevant characters are there to watch them.
In the fifth book, Ron, Neville, Ginny and Luna somehow manage to knock out Draco, Crabbe, Goyle and a couple of other Slytherins on the Hogwarts Express. Only the aftermath is described.
In the final chapter of the Warhammer 40,000: Gaunt's Ghosts novel Sabbat Martyr, the narrator speaks of a week-long battle supposedly more intense than any other recorded in the narrative, but we do not get to read it. Admittedly, it came on the heels of the final confrontation between Saint Sabbat and the Magister, but still...
It's also at the end of the book, just after the climax. Also...perhaps it's best not to dwell on a heroic battle that occurs after someone tells the people getting slaughtered that they might want to try fighting back.
Blacksmith Donal Noye leads a defense against a group of giants while Castle Black is under siege. The readers only get to see the aftermath: all the combatants and Donal and the giant king Mag killed each other.
Virtually all of Robb Stark's campaign in the Riverlands and Westerlands during the War of the Five Kings. The closest we get to "seeing" a battle is Catelyn listening to the battle of the Whispering Wood from a distance. The only times we see Robb "onpage" in the second and third books is when he's showing his father's aptitude for politics.
In Twilight, Bella passes out before we get to see the fight between Edward and James.
We hear that Leah told off Bella for being mean to Jacob, perhaps the only time in the series when anyone calls her out on anything, but never see it.
This trope is used masterfully in China Miéville's The Scar: the book's largely about a narrator 'lost at sea' in a city that's often outside the bounds of her knowledge and understanding. The story builds up a rivalry - and an intriguing past acquaintanceship - between two overpowered badasses, and they finally get their showdown a couple of chapters from the end...but then the fight scene gets skipped entirely, and instead we get to see the aftermath at the start of the next chapter.
The Wheel of Time series often has large, plot-central battles being stated to occur, but the only "on-screen" action being the commanders discussing the battles before, after, or away from the field. The most egregious example would be the fight between Mat and Couladin. Not only does one major character kill another offscreen, it actually narrates Mat preparing for the encounter, then cuts to the victory celebration. To be fair when battles are described, they are both epic and empty of Hollywood Tactics. However, if the movie adaptation that people are talking about does happen and does get to book 5, they had better include that fight onscreen.
Shogun ends just before the decisive, climactic battle that the whole book was building up to. A battle between hundreds of thousands of samurais that historically decided the fate of Japan.
After spending the entirety of The Hobbit running around pissing everyone off and fleeing the bad karma before they can get what's coming to them, Bilbo and friends are finally cornered, with everyone turned against them and each other. The book climaxes as huge armies of men, elves and dwarves are about to begin fighting a humongous battle when Gandalf comes out of nowhere and tells them to stop, because huge armies of goblins and wargs with bats are coming at them! They quickly strike a truce to fight the common foe, and an enormous battle rages! At the brink of defeat, sentient eagles suddenly show up out of nowhere to help! The Holy Shit Quotient is off the scale! Oh, wait, Bilbo just got bonked on the bean by a rock. At least he gets to hear about it later.
Gandalf's journey to Minas Tirith to study the One Ring, his imprisonment in and escape from Isengard, Boromir's last stand, the Ents wrecking Isengard, and the Dead Men driving off the Corsairs at Pelargir are described in detail by those involved in flashback. The Film of the Book depicts these events in real time.
Gandalf's battle with the Balrog in Moria also appears in flashbacks, though he only described it in the book; in the film it's actually shown.
After the Ring was destroyed, having fought off several attacks by Sauron's forces, the Elves of Lothlórien marched on Dol Guldur, Sauron's fortress in Mirkwood, and captured it. Then Galadriel came forth and "threw down its walls and laid bare its pits". We never get to 'see' this firsthand.
Also, in the Appendices it is stated that the largest battle of the war was not the Battle of Pelennor Fields (which gets a half-dozen chapters focused on it) but actually the Battle of Dale, which took place around the same time and involved far more combatants, but is only alluded to in the text. This is kind of justified though by the main theme of the book- that the war cannot be won by force of arms, and that only be destroying the Ring can Sauron be defeated.
The Dune novels suffer from Frank's annoying tendency to skip some amazing moments of awesome which... are actually rather crucial to the story. The Fremen/Sardaukar battle at the end of the original novel, for example, was alluded to in less than a page (fortunately the duel between Paul and Feyd-Rautha in the last chapter was dealt with in full); the massive jihad between the events of Dune and Dune Messiah is never covered... even the destruction of Arrakis at the end of Heretics of Dune, for example, was skipped over between chapters. Though what crowning moments of awesome are included more than make up for it.
Happens a bit in Patricia Wrede's Enchanted Forest Chronicles, though as a Trope Breaking semi-parody of fantasy and fairy tales in general, it was to be expected. Dealing with the evil fire witch and the averted epic battle between the dragons and wizards are two such moments.
Less understandably so, however, is the two-part series by Mark Acres, Dragonspawn and Dragonwar, presumably he is terrible at writing large battles as that's the only real explanation. The only large military engagement one sees is when Bagsby the thief rides in and blasts Ruprecht's army with his two dragons, shortening the fight considerably.
Happens so many times in The Iliad. There are prophecies delivered multiple times that if Achilles kills Hector, both Troy will fall and Achilles will die soon after, but both Troy and Achilles are still standing at the end of the book.
Two of the main villains from The Death Gate Cycle are Xar (a Magic KnightBadass Grandpa who is quite possibly the most powerful wizard who ever lived) and Kleitus (an undead, omnicidal necromancer who is magically weaker but makes up for it by being almost impossible to destroy). They meet, fight, and Xar manages to force Kleitus into being his (grudging) servant. Unfortunately, this all takes place off page- the reader is even told that the fight was spectacular, albeit brief, but never gets to see it. Now, seeing it wasn't plot-centric or anything, knowing what happened, but still- that would have been awesome. Not to mention how Alfred turns into a giant dragon and opens a can of whoopass on the creatures of the Labyrinth, including at least one evil dragon of the kind that had only ever been defeated once by aforementioned Xar. The fight never gets described.
The Dresden Files has had several major cases of this. In Dead Beat, Harry is told by Morgan, Luccio and Ramirez about the recent series of battles they fought in against the Red Court in which ONE HUNDRED AND FORTY SEVEN Wardens and thousands of vampires died.. To give you some sense of how big that was, there are only about 200 Wardens in all. The next book Proven Guilty nearly matches that with a massive battle at a Venatori Umborum base in Oregon in which Morgan nearly kills the Red King and Michael singlehandedly saves 40 Warden trainees at Luccio's boot camp.
Done deliberately in The Charterhouse of Parma by Stendhal: The main character joins up Napoleon's army dreaming of heroic deeds and epic action, but when the battle of Waterloo takes place, all he sees is smoke and confusion. The whole thing is a huge anticlimax for him, and, symbolically, it marks the passing of the Revolutionary era.
The Pardal plot in the last Empire from the Ashes book ends abruptly during the climactic final battle, with only a brief transmission in the last scene to indicate that they succeeded and did not, in fact, die horribly. Sean finding the right access code to bring down the defenses? Bringing the full might of Imperial technology on those sorry zealot asses in a well-deserved Curb-Stomp Battle? Taking total control of a global theocratic empire? Nope.
The Lion, the Witch, and the Wardrobe. So much. A huge battle between Aslan's forces and the White Witch's army is set up...cut to Lucy and Susan saving Aslan and riding him to the battle, where we see it mostly ended and Peter tells them "Hey, we just had a really cool battle, you missed most of it though".
The 2005 Walden-Media film rectified this, even giving the girls minor but pivotal roles at the battle's very end.
In Yahtzee Croshaw's (currently) only book, Mogworld, this happens twice, for exactly the same reason each time... the main character, Jim, intentionally walks away from the horde of invaders threatening the mage's school where he's studying, and watches it from afar/death...
Gordon Korman's Losing Joe's Place averts and plays it straight in the same chapter. The three main characters see fellow roommate Rootbeer decimate six pro wrestlers one by one in an exhibition, only to have all six gang up on them in the parking lot later, As this fight is about to begin, the narrator is knocked out, only to come to and see all six wrestlers beaten and spread out across the pavement. One friend says, "It was like the end of the world, and you missed it!" while Rootbeer just says, "Those guys - they had bad luck."
The Hunger Games by Suzanne Collins is told in first-person, so much of the action in the arena happens "offscreen", including what is implied to be an epic two-day battle between the two strongest competitors in the arena. In the rain.
Peeta has a real badass moment that we never get to see, when he kills Brutus during the Quarter Quell in Catching Fire.
In Oath of Swords, while the narrator is watching Bahzell busy fighting Harnak as champion of Tomanak to Harnak's champion of Sharna, Brandark is caught in a melee against four hradani simultaneously — and kills three of the four before going down.
In The Island of Doctor Moreau, Moreau grudgingly offers a fragmentary account of how a limbless, writhing thing he'd created had killed one of his human servants and several of his other creations before being hunted down. Although this event sounds like it might make a decent adventure/horror tale all by itself, no details about the creature or its actions and demise are offered, and it's never mentioned again.
In the first book of the Queen's Thief series, the main character singlehandedly trounces an entire group of men on horseback, while he is on foot and armed only with a practice sword. The narration is vague and cuts away from the fight almost as soon as it begins. All the recap of the fight we get afterwards is all the onlookers later telling him how shocked and awed they were at his unexpected performance (despite that he technically lost the fight). This reaction is largely also true for the readers, who previously had no idea he even knew how to hold a sword. Possibly justified in that the narrator was trying not to be sick during most of the fight.
The Amber Spyglass has a large proportion of the sentient beings from the entire multiverse lining up for war against all the other sentient beings led by God, or the Metatron in the name of God following a coup several millennia ago. Do we get to see the epic clash of arms that would put even Norse myth to shame? Nope, two kids run across the battleground, dodge some cavalry, see an old friend and promptly leave for a different universe. There were enough POV characters involved in the battle, couldn't we have stuck with one of them for a bit?
In The Wishsong Of Shannara, Jair Ohmsford and his companion, Slanter, leave the area before we can see more than the beginning of the duel between Garret Jax, the Weapons Master, and the Jachyra, a maniacal Fae being, one of whom had previously slain Allanon. When they come back, Jax is dead and the Jachyra gone, but no one knows how the fight actually turned out.
Sherlock Holmes and Moriarty have an epic Battle of Wits that lasts either four months or a few years (the Sherlockian canon isn't known for its consistency.) We only catch the final week of the conflict after Holmes has backed Moriarty into a corner, and one incident in the prequel written twenty-one years later which, while very dramatic, doesn't appear to have much impact on the battle at large. Pastiche writers don't seem to want to fill in this gap, which is strange given how many of them have the requisite chutzpah to write Holmes-versus-Moriarty to begin with.
Sometime between The Honor of the Queen and The Short Victorious War, Admiral Hamish Alexander, Earl of White Haven, spectacularly won the Third Battle of Yeltsin, a decisive Manticoran victory that critically shaped the opening phases of the war. We don't see it.
While Abigail Hearns (plus a bunch of Marines) was busy killing pirates on the surface of the planet Refuge in "The Service of the Sword", Captain Michael Oversteegen won a three-on-one engagement with a trio of Solarian heavy cruisers, out of whom he blasted the complete crap despite having only a single similarly-sized ship himself. We don't see it, or in fact him until he re-enters the story by blowing the fourth heavy cruiser out of space with a single salvo. Unsurprisingly, "Tiberian" thereafter becomes Oversteegen's Remember When You Blew Up a Sun? moment.
Played as a kind of running gag in the Blackcollar series. Team member Mordecai is described possibly the best hand to hand fighter to come out of the Blackcollar program... which considering that the Blackcollars were already insanely dangerous Super Soldiers, he may be the greatest to have ever lived. But team newbie Caine has the running bad luck that he keeps missing Mordecai in action, like glancing down a hall for a few seconds, only to turn back and see that Mordecai has taken down four men without making a sound.
The Thrawn Trilogy alludes to a duel between Yoda and a rogue Jedi on Dagobah. And to anyone who's seen Yoda get dangerous, it couldn't be anything but awesome.
In the After the End book The Dog Stars, Higs returns from his trip to find that his farm had been ambushed by a large squad of raiders, and his partner Bangly had made a Last Stand to defend it. Higs eventually discovers that Bangley is actually still barely clinging to life, having killed the entire raider band.