Unless they're trying to make a point, even masters and servants in Duumvirate are on first name bases. The only exceptions are for people who hate their given names, or have a formal title and can't be told apart.
Eustace and Jill in C. S. Lewis's The Silver Chair call each other by last name at first, as was customary at their school. Toward the end of the book, as they became friends, they start calling each other by their first names.
In Harry Potter, Sirius Black was referred to by his last name until Harry started to become attached to him. Yet Lupin, of whom Harry is equally fond, is always Lupin, just as Hagrid is always Hagrid and Dumbledore is always Dumbledore.
Speaking of Dumbles, he calls everyone by their first name, whether he likes them or not. And whether they (*cough*Voldemort*cough*) like it or not. Of course, Dumbledore did know Voldemort before he was Voldemort.
On one occasion Professor "Mad-Eye" Moody makes a rude comment, only for Dumbledore to snap "Alastor!" Harry is momentarily confused, then realizes that "Mad-Eye" probably isn't Moody's real first name.
Interestingly, Harry always has to be prompted by others to call Snape "Professor," except in one instance - when he first uses what turns out to be his signature spell, Expelliarmus:
"Shouldn't have let Professor Snape teach us that one," Harry said grimly.
Throughout the series, Snape himself always addresses his colleague and former teacher as ProfessorMcGonagall; however, in Book Seven, after he becomes Headmaster, he calls her Minerva.
Also, Harry starts to call Voldemort "Riddle" during the final battle. It takes some cojones to do that.
And just before that he stops thinking of his future mother in law as Mrs Weasley, as referenced by the use of Molly.
In Robert Heinlein's Double Star, the emperor realizes the Emergency Impersonation imposture when the double calls him by title privately; they had been on a first name basis in private.
In Death Or Glory, when Felicia Tayber first calls him "Ciaphas," Amberley Vail speculates they had had some time to socialize — though when is the question. (There are other hints that they had a fling during the course of the book, including one infamous mention of a particular mechadendrite.)
In Duty Calls, the colonel of his regiment calls him "Ciaphas" to emphasize that she is personally concerned about him after a concussion and he should not act as if he had a Hard Head.
An old school friend calls him "Cai", which Cain does not like and does not take from anyone else — as a sign this friend is annoying and presumptuous.
In Cain's Last Stand, Cain gives information to an agent of Inquisitor Vail, and refers to her as "Amberley", to remind him that she and Cain are close.
When Amberley herself calls him "Ciaphas" in For The Emperor, he later reflects on how it seemed so natural that he didn't think to question it at the time. It's probably got something to do with Love at First Sight. Or because she's an Inquisitor.
In Dan Abnett's Gaunt's Ghosts novel Necropolis, Surgeon Ana Curth insisted on Surgeon Curth when she first met Dorden. At the end, after they had worked together the length of the war, she had comforted him while he wept over his son's death, and she decided to join the Ghosts, she tells Dorden about her decision, and
She smiled sadly. "I think, by now, that it's all right for you to call me Ana."
Most of the Gaunt's Ghosts are on a Last Name Basis, so that use of the first name, while not often as pointed as this, is usually significant sign of friendship or informality. (Except Dalin Criid — because "Criid" means Tona Criid, his adoptive mother.)
A more straight example: After Traitor General, Rawne and Guant address each other by first name, at least in private
In Journey's End Stanhope angrily says to his old school friend and subordinate Raleigh "Don't you Dennis me!". He later has a Pet the Dog moment at the end when he calls Raleigh by his first name after he's dying.
In Dorothy L. Sayers' Gaudy Night, Harriet Vane has an awkward moment: because she is addressing Lord Peter Wimsey before the senior members of the university, she (properly) calls him "Lord Peter" (as opposed to just "Peter") for the first time in a long time.
"Oh, then, Curdie, you must call me just Irene and no more." "No, indeed," said the nurse indignantly. "He shall do no such thing." "What shall he call me, then, Lootie?" "Your Royal Highness." "My Royal Highness! What's that? No, no, Lootie. I won't be called names. I don't like them. You told me once yourself it's only rude children that call names; and I'm sure Curdie wouldn't be rude. Curdie, my name's Irene."
In Dan Abnett's Warhammer 40,000Horus Heresy novel Horus Rising, when one iterator praises another for how her picture captures "Garviel", she observes that he must be a friend of Captain Loken, to call him so.
In Graham McNeill's False Gods, when Loken goes to speak with Erebus, Erebus insists on "Erebus" and not "First Chaplain". Later, Horus tells the remembrancer Petronella Vivar to call him "Horus" and not "my lord."
In Graham McNeill's Fulgrim, when Fayle requests permission to speak, Fulgrim addresses him by first name. His pleasure is visible, and one of Fulgrim's soldiers reflects on the skill with which Fulgrim flatters. Later, when Solomon Demeter tries to pry the true story of what happened on Murder out of Saul Tarvitz, he calls him "Saul" — and then asks permission.
At the party, Gwyneth Blair insists that Judd Cauley call her "Gwyneth" — after all, he did when they were children.
In Terry Pratchett's A Hat Full of Sky, Esmerelda Weatherwax, a highly respected witch, grants Tiffany the right to call her "Granny Weatherwax", usually reserved for those who are quite close to her while everyone else calls her Mistress Weatherwax.
And in Soul Music, the following conversation addresses the issue for someone whose acquaintance everyone eventually makes:
Nobby: What's his first name?
Colon: What do you mean? He's Death. Just Death!
Nobby: No, that's his job. What do his friends call him?
Nobby: Oh, right. Still, he must've got one, hasn't he?
Colon: You mean like, "Keith" Death?
Nobby: I think he looks like a Leonard.
In the case above, it was Susan. The leader of Ankh-Morpork's City Watch uses this trope in an odd way - he's usually fine with being referred to as "Commander Vimes," while he lets coppers he's worked with for a while use the more informal "Mister Vimes." But if Sergeant Colon ever calls him "Sam," he knows his old friend is worried.
Vimes himself is confused when Sybil asks him, "Did you go to see Havelock?" because he can't wrap his head around the Patrician having a first name, or anyone knowing him well enough to call him by it. (Only a few other people are ever seen calling him Havelock, mostly Mustrum and Hughnon Ridcully, and, oddly, Topsy Lavish.)
In the beginning of Truman Capote's In Cold Blood, Perry Smith and Dick Hickock are referred to as Perry and Dick, making them more relatable to the audience. Later, they are referred to as Smith and Hickock, which serves to distance them from the reader.
In The Wheel of Time, Aiel have trouble grasping the notion of a name with multiple parts, and so tend to be on a Full-Name Basis with wetlanders in general. However, Aviendha refers to Elayne by just her first name, which is as significant as it would be in Japanese.
Chiss have rather long, punctuated names like Mitth'raw'nuruodo. Their shorter "core names", like Thrawn, appear to be mostly reserved for close acquaintances, relatives, and friends. When Thrawn met Jorj Car'das in Outbound Flight he started on a Full-Name Basis and was told to call Car'das by his last name and, since Car'das hopelessly mangled Mitth'raw'nuruodo, allowed Car'das and the other two humans to use his core name. Later he asks his brother Mitth'ras'saffic if he will let them use his core name, Thrass, and Thrass initially refuses, but relents because the humans did save Thrawn's life, at least. When he came into Imperial service Thrawn simply went by Thrawn since just about everyone had trouble with his full name, to the point where the fact that he had more name was surprising.
By Survivor's Quest it seems like the Chiss interacting with representatives of the New Republic and the Empire of the Hand have acclimated; Chaf'orm'bintrano, who was suspicious and dismissive of humans fifty years ago, quickly tells Luke and Mara to call him Formbi, and gives the core names of every Chiss he introduces to them, much to Prard'ras'kleoni's annoyance.
Everyone else who lets me ride on their dinosaur calls me Carlos
In Cold Days Harry referring to Murphy more and more as Karrin signals a shift in their relationship towards a Relationship Upgrade even if it isn't achieved in the book.
In C.S. Goto's Blood Ravens trilogy, an eldar farseer calls Gabriel by his first name; he finds it presumptuous but doesn't object.
In C.S. Friedman's Coldfire Trilogy, Tarrant only calls Vryce by his first name twice. Both times are significant.
It also works the other way round: Tarrant is called "Gerald" by Damien Vryce only in a few and very important instances.
Kitty Norville refers to people by their last names in her narrative until they become friends or allies. Then she calls them by their first names.
Mercy Thompson refers to people by their first name if they're pack or friends of the pack. Enemies or unknown quantities are referred to by last name only.
Kate, the Demon Hunting Soccer Mom also refers to friends by first names, mostly. In the second book in the series, she referred to one of the bad guys by his first name — but he was a celebrity whose first name was a household word of sorts.
In Quantum Gravity, elf culture goes from second half of name to first half of name. This is important, as everything except the first half of one's name is on the books, and elves have stronger True Names than any other being.
Tath, specifically, asks Lila to switch to Ilya after becoming closer.
Interestingly, even after They Do, Zal still insists upon Zal, not Azrezal. Could be because, being also demonic, he thinks of it as a nickname, rather than a more formal address.
Sarah Waters has a very neat trick in Affinity, which is made up of two diaries. In the main narrative, the protagonist sometimes refers to her maid Vigers. In the other, mention is made of a character called Ruth. They are in fact the same person. The reader only discovers this in the very last pages, and the fact has terrible consequences.
In Edgar Rice Burroughs's The Mad King, Emma recounts when a child, the prince had insisted on her calling him "Leopold" and made her kiss him every time she called him "highness."
A Song of Ice and Fire has a variation with Jaime and Brienne. Initially they refer to eachother with derogatory nicknames ("wench"/"Kingslayer"), but as they travel together they begin to develop respect for each other as knights and begin to use formal honorifics ("ser"/"my lady") and eventually actual names, though Jaime reserves "wench" as a term of affection/frustration when Brienne is being particularly honourable. Brienne refers to Jaime almost exclusively as "Ser Jaime" in A Feast for Crows, and when Jaime runs into her ex-fiance, who badmouths Brienne, he makes him address her with respect, backed up by his right hand.
“The bear was less hairy than that freak, I’ll—”
“You are speaking of a highborn lady, ser. Call her by her name.”
In Drei Männer im Schnee by Erich Kästner, Hagedorn and Tobler become fast friends, and shift to a first-name basis quite late in the narrative. This is a significant event, and not something that was taken lightly in German culture at the time.
In Jane Austen people are only on a first name basis when they are intimate friends.
In Pride and Prejudice it is considered a bit precipitate that of Lydia's three weeks' friendship with the Colonel's new wife, they have been intimate [on a first name basis] for two of those weeks.
In Northanger Abbey, the speed with which Catherine and Isabella call each other by first name is noted.
The progress of the friendship between Catherine and Isabella was quick as its beginning had been warm, and they passed so rapidly through every gradation of increasing tenderness that there was shortly no fresh proof of it to be given to their friends or themselves. They called each other by their Christian name, were always arm in arm when they walked, pinned up each other's train for the dance, and were not to be divided in the set
There is exactly one person who calls Sherlock Holmes "Sherlock," and that is his older brother Mycroft. Perhaps not a mark of affection so much as it being a bit odd to refer to someone by the surname you share.
In P. G. Wodehouse's Jill The Reckless, Derek is annoyed that Jill is on a first-name basis with Wally Mason; she explains that they knew each other as children.
In George MacDonald's "Port In A Storm", the father stops his story to comment that he and their mother were on first name basis already at this point in the story.
In E. D. Baker's The Wide-Awake Princess, Liam takes to calling Annie by her first name while they are searching. Annie doesn't even think about it until she has a minute to reflect. Later, when he calls her "Your Highness" she notes he must be doing it to alert the prince to her being a princess.
Played With in Animorphs—-the characters repeatedly say that they won't reveal their last names in the narrative for fear of being discovered by the Yeerks whom they secretly fighting. It's only in the penultimate book that we discover that Jake (and presumably his cousin Rachel) have the surname Berenson. The one time a surname has to be used in dialogue it's censored: "Tobias ———." (Fanon says its "Fangor," but probably not since he had a different legal father.)
When President Pritchart and Queen Elizabeth III start calling each other "Elizabeth" and "Eloise", the Solarian League and the Mesan Alignment are in deep shit.
But when Honor Harrington and Thomas Theisman start throwing around "Honor" and "Tom", the Solarian League and the Mesan Alignment are fucked.
In Bernard Cornwell's Sharpe's Tiger, Sharpe bluntly explains to Lawford that they are "Dick" and "Bill" because they are privates, deserting together.
Invoked and subverted in the Myth Adventures book "M.Y.T.H. Inc In Action", when Guido and Nunzio are being put through basic training by a Drill Sergeant Nasty. "The first thing you should know is that we are on a first name basis here... and my first name is SERGEANT!"
A very subtle variant in The Lord of the Rings. Westron, the common tongue that Tolkien translated into English, has two different forms of the second person pronoun: a "familiar" form for use with family and close friends, and a "formal" form for everyone else. However, the Hobbit dialect of Westron somehow dropped the "formal" form entirely, using the "familiar" form for everyone. This gives those in Gondor who listen to Pippin's conversations the impression that he is a close personal friend of such notables as the Steward, the Captain Of the Guard, etc.
In the chapter after, it is mentioned that this has led to Pippin having to repeatedly debunk rumors that he is a prince among his people (technically true, since his father is Thain of the Shire, but the title has no meaning when the Shire isn't being invaded), that he is there to negotiate an alliance, and that there is a hobbit army marching to reinforce Minas Tirith. Apparently someone heard him use informal pronouns when speaking to Denethor, assumed no-one who wasn't at least a prince would dare to adress the Steward of Gondor so informally at a first meeting, and everything just spun off from there.
In The Underland Chronicles, Gregor, Boots and Lizzie's last name is never mentioned. Their mother's first name is Grace.