Every narrator is unreliable to some degree — human perception is selective, memory is reconstructive, and narration is always a choice about what to include and what to omit. What distinguishes the unreliable narrator as a literary device is the degree to which the gap between what the narrator reports and what the reader understands actually happened is made structurally visible. The best novels built around unreliable narrators use that gap to do specific work: to represent forms of self-deception, psychological defense, or structural social blindness that could not be shown in any other way.
The Remains of the Day by Kazuo Ishiguro is probably the most fully realized study of unreliable narration in contemporary fiction. Stevens, the butler, spends the entire novel persuading himself — and initially the reader — that his decades of devoted service to a fascist-sympathizing lord were a form of professional dignity. Every claim he makes about "the great houses of England" and "dignity" and the proper relationship between a servant and his employer is a way of not saying what the reader gradually understands: that he has wasted his life, that he knew it and couldn't acknowledge it, and that the woman he loved is now beyond his reach. Ishiguro gives Stevens the most meticulous, precise prose — the voice of a man who controls everything, including what he allows himself to perceive.
The Stranger by Albert Camus is unreliable in a different register. Meursault does not lie in the conventional sense — he simply fails to process his experience through the categories of emotion and social meaning that other people apply to it. He reports his mother's death, the day on the beach, the shooting, with the same flat affectlessness. The reader must supply the significance. Camus is not showing a man who is hiding something; he is showing a man from whom something is genuinely absent, and the horror of the novel is that his indifference is, under certain lights, more honest than the social performance of grief and significance that other characters (and the reader) perform without examining.
Vladimir Nabokov's Lolita (1955) is the most instructive example of unreliable narration as formal defense. Humbert Humbert is the most accomplished prose stylist in the novel and uses that skill entirely in the service of his own exculpation. He is charming, learned, funny, and a predator. Nabokov gives him the language to make his case because the reader's job is to read through the language to what the language is concealing. The novel is a trap for readers who mistake prose beauty for moral authority — which is exactly the trap Humbert sets, and which Nabokov intended readers to notice themselves falling into.
Gillian Flynn's Gone Girl (2012) used unreliable narration in a more popular register but with genuine structural craft: both narrators are unreliable and the reader must triangulate between them. The novel's particular achievement is making both unreliabilities plausible — Nick's selective memory of his marriage, Amy's deliberate performance for her own diary — without either reading as simply lying. They are each the protagonist of their own story and the antagonist of the other's, which is a version of how unreliability actually works in relationships.
Ford Madox Ford's The Good Soldier (1915) is the novel that most clearly announced what unreliable narration could achieve. John Dowell narrates the story of two couples and the adulterous relationship at the center of it, but his narration circles and backtracks and contradicts itself in ways that gradually reveal him to be the least reliable witness to his own life. Ford used the term "the best French novel in English" to describe it — what he meant was that it used narration the way a French novelist would: not to deliver information but to reveal character through the act of narration itself.
The common thread in these novels is not mystery or deception, though both can be present. It is the choice to make the narrating consciousness itself the subject, alongside whatever plot is being narrated. When an author uses an unreliable narrator well, the reader is simultaneously inside the narrator's perspective and watching from outside it — a position of double awareness that reliable narration can approximate but never quite reproduce.