In Nancy Drew (and the very similar The Hardy Boys), the protagonists and their friends were knocked unconscious multiple times in every book. It apparently didn't cause any lasting damage: Nancy never lost her detecting touch.
Subverted in Sandy Mitchell's Ciaphas Cain novel Duty Calls. Cain suffers a realistic head injury, putting him out for three days and inspiring nausea and dizziness. Cain plays it up or down as suits him, causing other characters to remind him that such an injury is serious.
Played half straight in For the Emperor, where Jurgen takes a glancing hit to the head. His carapace armor helmet helps protect him, particularly since the bolter round detonates prematurely (it's supposed to penetrate flesh and then go off; detonating on the wrong side of armor robs it of half its force). On the other hand, most headshots with bolters are fatal regardless of armor, so clearly he was naturally tough as well as armored.
This receives a Call Forward in Death or Glory, where Jurgen is tossed about in a crash-landing escape pod and suffers a minor bruise as a result. Cain mentions wryly that Jurgen's head is probably too thick to be cracked by anything short of a bolter round.
In William King's Warhammer 40,000 novel Space Wolf, when Ragnor considers that injuries can be more severe than they look, he remembers stories of men who suffered a light blow to the head, fought through a battle — and dropped dead.
Subverted and lampshaded in Dorothy L Sayers's Murder Must Advertise; Lord Peter Wimsey comments on how the hero in an in-universe piece of fiction lightly shrugs off all injuries, while Charles Parker has been incapacitated by one head injury.
Discworld, Maskerade: Nanny Ogg is hit over the head with a champagne bottle. It does stun her, but she recovers quite quickly because there is some Dwarf in the Ogg ancestry.
Earlier, in Witches Abroad, Nanny has a house land on her. Turns out her witch-hat is reinforced, and Nanny survives because all the hat had to do was bust through the rotton floorboards of the lowest floor.
Averted in Monstrous Regiment: the medically-trained Igorina very carefully taps a guard's head in the right spot to knock him out without badly injuring him. (It helps that he stands still to let her do it, wanting it to seem like he put up a fight against the title regiment.)
Invoked in Night Watch, where Vimes stops his younger self from hitting someone on the head in a sneak attack due to him knowing the risk of it killing the man.
Kahlan, in Sword of Truth, becomes determined to sneak out of a palace at one point, and is very reluctant to hit any guard who might insist she stay inside for her own safety, as she knows that she could as easily kill them as knock them out.
In the two-part Doctor WhoEighth Doctor Adventures novel Interference, Sarah Jane lampshaded this, by pointing out the risk of severe damage, and asks an alien exactly how they manage to do it without inflicting said severe damage, every damn time.
In P.C. Hodgell's Chronicles of the Kencyrath, heroine Jame is hit over the head at least twice per book, and recovers just fine. However, she's not human, and is or a race with a powerful Healing Factor; furthermore, even with that, she suffers realistic concussion effects from the worse beatings (ones that a normal human would have died from).
Played straight in the works of German author Karl May, where a knockout blow to the head is practically the Author Avatar's signature move. (His nom de guerre in the fictional West is "Old Shatterhand" for a reason.) Very occasionally subverted when a suitably nasty villain dropped by this does not come to his senses anymore... but usually it works just fine, in line with his stated aversion to killing other human beings.
Occasionally, he hits his targets too lightly, and they wake up too soon, usually adding humor to an otherwise suspenseful scene. Or it could make it even more suspenseful, if he ends up being caught by the enemy.
Played with all over the place in The Belgariad. Garion has a habit of hitting his head on things to no long-term effect, which eventually becomes a Running Gag and is even lampshaded by Belgarath. The heroes also knock out whole swathes of Mooks, who conveniently come to when it's necessary to interrogate them. It's averted in a few notable cases, however: King Korodullin gets brained at the battle of Thull Mardu and has difficulty hearing afterwards; later in the Malloreon, a Cherek assassin gets hit over the head with an axe and Polgara proclaims him incapable of being revived.
Averted in Susan Dexter's The Prince of Ill Luck. Leith, the eponymous unlucky prince, gets hit on the head twice during the course of the book; both times, it renders him so sick as to be incapacitated for several days following.
All of Susan Dexter's books seem to approach this one pretty well; in The Wind-Witch, Kellis is so incapacitated by a blow to the head that he is unable to escape his prison when his fellow pirates do, and remains sick and nearly crippled for the week that follows his release. Admittedly, he was also hit by cold iron, which has poisonous effects on shapeshifters.
In Otherland, a drinking game could be made from the number of times a scene transition occurs through one or more protagonists getting knocked out, then reviving later with no ill effects. They are Inside a Computer System, so it's all technically virtual, although Your Mind Makes It Real is in play and many characters do indeed die in Real Life from fatal injuries in the Grail Network.
Lampshaped in the The Dresden Files. The hero, Harry Dresden, tends to get very much beat up over the course of a book. He's been shot, bitten, had bones broken, intentionally wrecked, near explosions, tossed around by supernatural powerful creatures, burned, frozen, and almost dissected while he was still alive. Thanks to Functional Magic, he recovers from these — slowly, but he still continues to heal (at a normal human rate), until a wound's traces have gone away. Despite his injuries (he comments there are phonebooks smaller than his medical file, when he sees a doctor struggle to lift it), Harry has never broken his skull. He says he thinks someone snuck in and did an adamantium bone-coating on his skull somehow.
Though I walk through the valley of the shadow of trauma, I will fear no concussion.
Subverted in Justin Cronin's The Passage. When Michael is said to have been unconscious for several days but has no slurring, unfocused eyes, or signs of a concussion, it tips off Sara, a nurse, that something isn't right.
Averted in the Gone series. In the second book, Hunter is knocked out by a crowbar to the head. When he wakes up, he can barely speak coherently, and after receiving supernatural healing, he is left with permanent brain damage.
In The Wishsong of Shannara Garet Jax establishes himself as a Bad Ass when he rescues Jair from a group of gnome warriors, killing them all with a Simple Staff. All but the leader of the group that is, when they're inspecting the bodies Jax notices that the leader is unconscious but still alive, and comments that the gnome must have a particularly thick skull.
Justified in The Supernaturalist, when Cosmo has a metal plate implanted in his skull during surgery. It becomes a Chekhov's Gun later on when he needs to headbutt his way out of a containment vat.
In Galaxy of Fear, Tash gets headbutted in the stomach by her little brother Zak, who'd heard that they're headed to the Ithorian homeworld, and Ithorians are also known as Hammerheads. He cheerfully tells her that her stomach is much softer than the walls he'd been headbutting before finding her.
In J. R. R. Tolkien's The Hobbit, Bilbo is knocked unconscious both in the goblin tunnels and at the final battle. After the later, someone has to carry him down to the camp, but otherwise, he is fine.
In Journey to the West, Sun Wukong claims to have "iron head and bronze limbs" due to his overdose of immortality peaches and elixirs followed by a sojourn in Laozi's alchemical kiln. During the novel he's hit on the head by other demons who nevertheless fail to harm him, even when using swords.
Distinctly averted in the X-Wing Series, where getting clubbed in the head with a bottle (which didn't break) is enough to give Runt (one of the toughest members of Wraith Squadron) a concussion and put him out of the fight. Later invoked with when the Wraiths use this same trick on one of their own (in order to impress some enemies who are there), except that they're using a bottle made of fake breakaway glass, that shatters but leaves him ready for a brawl.
Most characters in Dinoverse, when they end up in the bodies of dinosaurs, gain much thicker, more durable skulls and can endure things like falling rocks much more easily.