In Aimee, Aimee is developed entirely through flashbacks, even though she killed herself before the events of the novel started.
Elfangor, the Prince that gave the Animorphs their powers, got a full fleshing-out in the prequel companion novel The Andalite Chronicles, including the revelation that he fathered one of the Animorphs.
In Aunt Dimity's Death, Lori's mother Beth is revealed in Lori's recollections and a thorough review of her decades-long correspondence with Dimity.
In Ann Neville's Barber Black Sheep, the main character's parents and little sister are dead before the novel begins, appearing only in flashbacks. The impact of their deaths on Oliver, the protagonist, is brought up throughout the novel.
In The Belgariad, Garion's parents and Polgara's sister start as clues on Garion's identity.
Ben Safford Mysteries: Elise's late husband is frequently mentioned but died of a heart attack years ago.
Sir Arthur Jennings, one of the many narrators of Marlon James' A Brief History Of Seven Killings, was assassinated several years before the novel's main action begins. Jacob Marley Apparel is the order of the day for ghosts in James' novel, so Sir Arthur is often frustrated with his multiple broken bones. He also appears to several other characters right before they're murdered themselves.
Keifer Porter may have been dead for six years, but damned if his presence isn't still felt and his actions aren't still causing problems.
On the positive side, there is Jerin's grandfather, who taught him a lot, and also was a prince, which becomes important in the plot, and the grandmothers, thanks to whom Jerin has many of the abilities of a trained spy.
The main character of the Burke series by Andrew Vachss often mentions his friend Wesley, who only appears in the fourth book, in which he apparently dies. He leaves no body behind and many who knew of him refuse to believe that he is dead, which Burke takes advantage of many times.
Terminal has Melissa Turnbridge, murdered 30 years before the book's events. As the opposition for the book includes the three then-teens who raped and apparently murdered her, as well as the man who tried to hide the body, this inevitably comes up.
The Burning Kingdoms: Malini's childhood friends/handmaids Alori and Narina are dead when the story begins. However, they appear in her flashbacks and thoughts often, as she continues to mourn them. Their loss motivates her to fight her brother Chandra, who had ordered their deaths.
Catch-22 has several, most notably Mudd, the dead man in Yossarian's tent.
Cat's Cradle has Dr. Felix Hoenikker, a fictional father of the atom bomb, who is of great interest to the protagonist, John, as a result of that; John strongly believes them to be a member of the same karass. His attempts to learn more about the man by interviewing his children, which eventually leads to him witnessing the end of the world, caused by another of Dr. Hoenikker's creations.
To a lesser extent, Cpl. Earl McCabe, a prominent figure in the backstory of Boknonism and the man who ruled San Lorenzo prior to "Papa" Monzano taking the throne.
Chocoholic Mysteries: "Kidnapping Clue" features Lee's uncle Phil (her mother's brother), who's in direct charge of his and his wife's chocolate shop. Cat Caper, picking up twelve years later, reveals that Phil had died in a car accident (caused by a drunk driver) eighteen months before the book starts, but he remains an important part of the background throughout the series.
A Christmas Carol: "Marley was dead: to begin with. There is no doubt whatever about that." Marley sets off the entire plot of the book, but Dickens wants you to know that Marley is very definitely dead before he ever makes an appearance.
Gaius Septimus in the Codex Alera series. In this first book, his history and death was mentioned in an offhand way, to explain why there is a succession crisis and give the archetypal Farm Boy protagonist a memorial to shelter from a storm in. By the end of the series we've learned that he was married, and to whom, who his friends were, and a lot abou what he was like.
In Stephen King's The Dark Tower series, almost everyone from Roland's past is one of these, including every character from the Wizard and Glass Mejis flashback (which is the majority of the book), aside from Roland, Walter and Sheemie.
In The Death Of The Necromancer, the titular Necromancer actually suffered the titular death about two hundred years before the book began. And they don't actually kill him again, they just put him into a permanent sleep.
Granny Aching from the Tiffany Aching series. Dead before the first book began, we only see her through Tiffany's memories. A bit of a shame since it would have been interesting to see a meeting between Granny Weatherwax and her. Pratchett has said in talks, "She's one of my best characters ever, and she's dead!"
In several books, the witches talk about the infamous Black Aliss, baddest of bad witches.
Night Watch by Terry Pratchett has a fairly interesting version of this trope. John Keel, Sam Vimes' mentor, is murdered shortly after Vimes stumbles back in time; he's mentioned at the beginning on the anniversary of his death (and the other soldiers). So Vimes ends up having to take his place in history, letting us see something similar to the story of John Keel, but not quite (note that it's not a Stable Time Loop; Vimes wasn't supposed to come back in time. He really was mentored by the real Keel, who was murdered due to Carcer being brought back in time as well. So John Keel's feats were really done by John Keel in the first timeline, but in this separate timeline, they're done by Vimes).
Laia Odo in The Dispossessed. She's been dead for several hundred years, but the entire world of the story is her creation, and the characters speak of her often. The short story The Day Before the Revolution, published after The Dispossessed subverts this by taking place before Odo's death and focusing on her.
Both of Harry Dresden's parents in The Dresden Files have appeared to him in various books, despite both being long dead by the time of the first book. His father has popped up in dreams, while his mother left him pre-recorded messages in his and his half-brother Thomas' souls in order to leave proof for her sons that they were brothers.
Heinrich Kemmler is also a major player in several books, in spite of the fact that he has been dead for over fifty years when the series begins. Notably, his research assistant and several of his apprentices play major parts in the story.
In Fatal Reunion for most of the book we are lead to believe Chaz Michaels is the Big Bad who is out causing most of the problems. It turns out he had long since been dead shortly after the robbery that lead to Piper leaving Memphis.
In The Father Luke Wolfe Trilogy, Janey Peer only appears in Flashbacks, and "Chollie" is the addressee of letters Father Wolfe writes in diary form. Janey turns out to still be alive, however.
Isaac Asimov's Foundation: After year 1 of the Foundation Era, Hari Seldon only appears from a holographic recording, but the Psychohistory he developed drives the entire Myth Arc. The Prequel stories go back to when he was alive, and describe his adventures in creating Psychohistory.
The Girl from the Miracles District has Johann, who's developed a lot in the second book even though he's been dead for years by the time the series begins.
Rosemary's suicide took place years before The Giver starts.
Go to Sleep (A Jeff the Killer Rewrite): Ben dies before the story's events, but it greatly impacts Jeff since they were best friends, and explains his hatred of Randy, who pushed Ben into the lake and caused him to drown. Ben reappears as a ghost with a grown-up Jeff to get back at Randy.
Lily and James Potter are killed at the very beginning of the series. Everything that we learn about them is through flashbacks or their magical equivalent.
Ariana Dumbledore plays a huge role in the Backstory despite having been dead for about a century and Harry never previously knowing she even existed.
In a literal example, Bathilda Bagshot is actually dead the entire time Harry and Hermione spend with her. Nagini is controlling her body but it's never stated how long she's been dead.
Regulus Black's story about leaving the Death Eaters and trying to stop Voldemort on his own is revealed in this book. He died right around the time Harry was born.
Many of the Order of the Phoenix members and the Death Eaters from the first war: Marlene McKinnon, Mrs. Weasley's brothers Gideon and Fabian Prewett, Edgar Bones, Caradoc Dearborn, Dorcas Meadowes, Benjy Fenwick, Evan Rosier, and Wilkes.
Harry Potter and the Half-Blood Prince reveals the story regarding Marvolo, Morfin, and Merope Gaunt, and Tom Riddle, Sr., who are Voldemort's maternal grandfather, uncle, mother, and father, respectively.
Phineas Nigellus Black, a former headmaster of Hogwarts, plays a pretty significant role in the last three books via his portraits in Hogwarts and Grimmauld Place.
Holes by Louis Sachar has an interesting take on this. The main story involves Stanley Yelnats at Camp Green Lake in the present day, with the other two stories of Kissin' Kate Barlow and Elya Yelnats taking place far before the events in the present. The characters in the latter two stories are of course dead by the time Stanley's story takes place, so arguably, you're seeing them as flashbacks or snippets of the past (though they aren't framed as such).
Hollow Kingdom (2019): Big Jim, the owner of Dennis and main character S.T., is only alive for the first two pages. However, S.T. not properly understanding what's become of him leads to the first major conflict of the narrative, and the crow's perspective of humanity being shaped by him factors into many of his decisions. Big Jim's character is also explored by means of anecdotes from S.T.
From The Hunger Games trilogy, Katniss's father. He died in a mine explosion 4 years before the first book, but he's still an important figure in her life. Over the course of the books, she tells of her childhood with him and reveals more of the circumstances immediately following the explosion and his death. It was even him who taught her about nightlock berries... In Catching Fire, we learn about Maysilee Donnor, who was a District 12 tribute along with Haymitch in the 2nd Quarter Quell. It turns out that she was friends with Katniss's mother and Madge's mother's twin sister. It was her pin that Madge gave to Katniss and went on to become the symbol of a revolution.
Crispin Salvador from Ilustrado. The main character's quest is to find out why Crispin died by piecing together Crispin's works and looking for his supposed life's work — The Bridges Ablaze. Then it's weirdly subverted at the end since it turns out that Crispin was writing the whole thing.
Eunice Branca in Robert A. Heinlein's I Will Fear No Evil After she is killed and Johann's brain is transplanted into her body, he finds her mind still present. It is left ambiguous whether she is actually still there or whether Johann is hallucinating her continued existence.
Julia's Kitchen opens with Cara finding out that her mother Julia and her sister Janie both died in a House Fire. Both receive a decent amount of characterization from Cara's memories.
The main characters father from The Land of Stories died a year before the events of the first book.
King's Lisey’s Story is a fine example as well. Scott Landon is a major character but is already deceased when the story opens.
Roger Kemp, previous owner of the Smith house and the time portal, in Locksmiths Closet. He disappeared several years before the beginning of the story, but it was his actions that set everything in motion.
Love Anthony opens about a year after Anthony's death, with Olivia still mourning him.
The Lovely Bones is narrated by a girl who was brutally murdered and observes from the afterlife her family dealing with her death and searching for her killer.
Matilda has Miss Honey's mother and father, the former of which died in childbirth, the latter of which was murdered by her aunt, Miss Trunchbull. They get considerably more backstory development in The Musical.
In Miracle Creek, Kitt and Henry are both killed offscreen in the first chapter. Kitt gets some paraphrased dialogue, although she isn't named, but Henry isn't even mentioned until after his death. Both are characterized entirely from other people's memories, especially Elizabeth's.
Mistborn has a few. The original series has Kwaan, who is the writer of the epigraphs, and Alendi, who Kwaan believed to be the hero of ages. While the sequel both subverts the trope and plays it straight. It mentions many of the characters from the first trilogy, as their legacies are remembered in the sequel, and also has Lessie, who was Wax's love interest before the beginning of the story. That he accidentally killed.
Jiguro, Balsa's foster father from Moribito: Guardian of the Spirit. Balsa tries to do right what he did right and avoid what he did wrong with her charge and the warriors hunting them.
Despite being dead for years at the beginning of the series, we learn a lot about Stephen Herondale through recollections and his relationships with other characters.
Celine Herondale's suicide takes place long before the story proper, but her relationships with current characters make her important.
In Number the Stars, the protagonist's deceased older sister, Lise. She died in a car accident while working for the Danish Resistance. Her fiancé, Peter, is still close with her family. At one point Ellen, the Jewish girl her family is hiding from the Nazis, is saved when her parents pass her off as Lise, the third daughter whom they have evidence exists.
For much of Vivien's side of things in The Obituary Writer, she refuses to believe the very likely possibility that her paramour David died during the Great San Francisco Earthquake, largely due to how he remains with her in her memories and left such an impact on her life. Unfortunately for her, her search eventually reveals that he was indeed killed during said disaster.
Ordinary People: Two in the novel: Buck, as recalled frequently in the novel by his father and brother, and shown in brief flashbacks in the film. In the novel, Calvin also frequently recalls his relationship with Arnold Bacon, an older mentor who guided him from an orphanage to law school, until they had a falling out over him marrying Beth. He never reconciled before the older man died.
In the backstory to Överenskommelser by Simona Ahrnstedt, Aurore Löwenström left her unhappy marriage to study abroad. But this meant that she also had to abandon her two young sons, and it led to one of them (Vilhelm) developing a pathological hatred against the growing feminist movement. He also became abusive towards his wife, his children, and his niece. Beatrice eventually becomes her uncle Vilhelm's favorite victim, because she reminds him so much of his mother/her grandmother. Aurore has been dead for years when the story starts, so she can never appear "on screen". But it becomes clear that her choice to abandon her sons to become independent, no matter how understandable it might be to Beatrice and plenty of modern readers, is what turned the Löwenströms into a Big, Screwed-Up Family.
In Percy Jackson and the Olympians, Thalia sacrificed herself to save others, and was turned into a tree just before her death. Said tree protects Camp Half-Blood, and when it's poisoned in the second book, the plot revolves around finding something to cure it. Which turns out to be an Evil Plan so that Thalia will be healed back to normal form, giving Kronos some other important half-blood to influence. He even goes, more or less, "Exactly as planned" just before the reveal. Thalia becomes a regular character from the third book on.
Niclays is haunted by the memory of his lost love, Jannart, whose death of the draconic plague sent Niclays into a tailspin that culminated in him squandering a queen's payment for the Elixir of Life and his exile to the East. Niclays is constantly thinking back to their days together, nagged by the thought of what Jannart would think of his worse acts in the story, and he finally realizes that the reasons for Jannart's death is tied both to the elixir and to efforts to prevent the Nameless One's return.
Queen Rosarian was killed when her daughter, the current Sabran IX, was just fourteen. Her death casts a long shadow over Sabran and is linked to the present threats against Sabran's life. There are still persistent rumors that she took the pirate captain Gian Harlowe as a lover (and that Sabran may be his daughter, not her "official" father's) which Gian's manner confirms whenever he speaks of her.
The lives of Cleolind and Neporo, the founder of the Priory and the woman who ate of the legendary mulberry tree, respectively, become increasingly important as the protagonists scour history to find a way to permanently stop the Nameless One.
The Queen of Ieflaria: Prince Albion, whom Princess Esofi was originally betrothed to, died before the start of the book. His death is what sparks the plot as she then agrees to marry his sister Adale instead. Everyone liked him, with Esofi remembering Albion fondly and Adale loving her brother, leaving her deeply saddened by his death. They bond when Esofi shares letters that she exchanged with Albion, making Adale and her realize the two have a mutual attraction, meaning their arranged marriage becomes a happy one.
The Quiet American begins with the narrator discovering the other main character, Pyle, is dead.
In the Raffles stories, the titular character is this, as the books are his friend Bunny's memoirs.
Ravensong: Nora, whose funeral happens in the first chapter.
Rebecca, a 1938 novel later made into a movie directed by Alfred Hitchcock. When the story opens, Rebecca (wife of the wealthy Maximilian de Winter) has been dead for a year already — but even in her absence, her presence is inescapable, as her memory casts its shadow over the entire story.
In most of the Redwall books, Martin the Warrior is long dead, but his spirit endures in Mossflower and he acts as a Magical Guide to many young warriors throughout the series, often contacting his chosen through dreams or speaking in someone's mind at a critical juncture. As one of the founding members of Redwall Abbey, he's a legendary figure in Mossflower's history, and most who grow up at the abbey are at least peripherally aware that he retains a mystical influence — characters will sometimes express hope for his aid in times of need, and someone claiming to have been visited by him in a dream is likely to be taken seriously. He does appear as a living character in Martin the Warrior, Mossflower, and The Legend of Luke, the three books that detail his personal story, but the events of these books take place years before most of the rest of series.
Reign of the Seven Spellblades: The first volume opens on a Distant Prologue where a woman flees several attackers only to be stabbed In the Back by another woman she thought was her rescuer. This is Chloe Halford, whose Plot-Triggering Death drives the Myth Arc and who frequently reappears in flashbacks and other characters' recollections, because she's secretly the mother of the main protagonist, Oliver Horn.
Requiem For A Ruler Of Worlds by Brian Daley. Cazpahr Weir, lord of a small interstellar empire, dies in the prologue. The main story concerns a hapless Terran bureaucrat left a mysterious bequest in Weir's will, for no reason he can (at first) understand.
Pei Kau-yug and Pei Shan-Wei from Safehold. Both briefly appear in the prologue, but die before main story begins. They did, however, send Nimue Alban to Safehold and hid her there, which is why the story's happening in the first place. Moreover, Kau Yung is often mentioned in Nimue's narration, as he was like a father to her, and Kau-yung and Shan-wei are considered the two worst demons in Safeholdian theology, which makes their names appear a lot, especially Shan-Wei's.
In Saving Zoë, Zoë is developed through her diary that her sister Echo is reading.
Beatrice, the Baudelaire parents (see previous), Jacques Snicket (technically, he was killed halfway through the series in the same book in which he was introduced) and probably several others in A Series of Unfortunate Events.
In Sion Crossing by Anthony Price, Bill Macallan has a significant influence on the course of the plot but turns out to have been dead all along.
In Six of Crows, Kaz Brekker hates Pekka Rollins because he blames him for the death of his brother, Jordie Rietveld, who died eight years before the book starts.
Someone Else's War: The entire novel is kickstarted by a boy trying to find his missing little brother. Said missing little brother turns out to have been dead since chapter one.
A Song of Ice and Fire: Among the most notable are Jon Arryn, Lyanna Stark, Brandon Stark, Rickard Stark, Rhaegar Targaryen, and his father Aerys.
In Soon I Will Be Invincible The Pharaoh (the villain one) was killed by CoreFire before Doctor Impossible escapes.
In the Spaceforce (2012) series, the Taysan agent Mizal is killed before the start of the second book (we never meet her). Her death is one of the driving motives behind the actions of several characters throughout the third book, including Salthar who was fruitlessly in love with her and Jay, who is out to avenge her. We hear her voice strongly in her unfinished field report.
In the Hand of Thrawn duology, Thrawn has been dead for ten years. A Big Bad Triumvirate conspires to make him appear to be Back from the Dead. Whenever the physical impersonator is in character and doing the impersonating the narration calls him Thrawn — as in "Thrawn inclined his head" — instead of his usual name, Flim. One of the triumvirate was cloned with a piece of Thrawn's mind, and there is an unfinished clone of his, but he doesn't appear himself. He's dead. But his presence is all throughout the duology; particularly with the actual Hand of Thrawn.
In The Thrawn Trilogy, Emperor Palpatine serves as this to Mara Jade, having implanted a subliminal command to KILL LUKE SKYWALKER into her brain as his final act. The command's effects on her, and her attempts to alternately fulfill and defy it, drive her character for the trilogy's second half.
Star Wars Kenobi: Sharad Hett, a former Jedi who joined a clan of Tusken Raiders, and who died some 13 years before the story begins, is an important element, being A'Yark's exposure to Jedi and the Force as well as a common acquintance for her and Ben.
Stepping on the Cracks: Barbara's husband Butch and Harold Bedford (the son of Margaret's neighbors) are both repeatedly mentioned as soldiers who've died in the war over the past year.
Sweep: The Story of a Girl and her Monster: The Sweep, Nan's previous guardian, has been absent from her life for five years at the time of the story, having walked out on her, leaving behind only the clump of soot that would become Charlie. Most of Nan's interactions with him take place in Flashbacks. We eventually learn he left because he didn't want Nan's final memories to be of a frail, dying old man.
Sweet & Bitter Magic: Marlena, Tamsin's twin sister, is dead but her death hangs over Tamsin as this was indirectly her fault, instigating the story since it also was how she'd gotten to where she's stuck at the beginning. Tamsin later also gets Marlena's diary, with her learning about what Marlena thought of her (it wasn't good). However, it turns out Marlena's alive.
"Talma Gordon": Some of the people involved in Thornton's story were either dead by the time it began (e.g. Isabel) or are dead by the present day (e.g. the murder victims).
Teen Power Inc.: Many books give a lot of prominence to someone who died before the book began.
The Ghost of Raven Hill has Elmo's grandfather, who died in a burglary prompted by how You Know Too Much.
The last act of Breaking Point teases the possibility that a past resident of the boarding house is buried in the cellar, which is in fact true. It just isn't the resident who everyone thinks is dead.
In Crime in the Picture, the late painter Selwyn Beard is constantly mentioned, with his paintings and still-living sister being large presences in the book.
The Tenets of Futilism has Sasha's father, Chris. While his effects upon his daughter's personality and beliefs are immense, he's dead before the opening of the novel and is described only through said daughter.
In Three Days to Never, Lisa Marrity dies on the first page, and her father has been dead for thirty years, but the consequences of their actions drive the whole plot.
Treasure Island: Captain Flint, who died prior to the events of the novel but whose actions are central to the plot.
Midway through Milan Kundera's The Unbearable Lightness of Being, we learn that Tomas and Tereza, the two main characters, have died in a road accident. Yet, due to the novel's non-linear narrative, we continue to follow their storyline up to the end of the novel. Since the film arranged the plot more linearly, it had to save that Reveal for the penultimate scene.
Under Heaven: General Shen Gao, Shen Tai's father. At the beginning of the book, he's been dead for years; indeed, Tai is nearly at the end of a two year 'mourning period' due to his death. Most of his accomplishments and opinions (including personal Character Development) are provided by narration, or Tai's self-reflection.
An Unkindness of Ghosts: Aster's mother Lune Grey died the day Aster was born twenty-five years ago. She is a near-legendary figure, and Aster spends much time poring over the notes she left, which are full of encoded information about their Generation Ship and its future.
We only hear of Andre Dragomir, the brother of Lissa, following his death in a car accident. He was a loving brother, a popular guy, and a jerk in his treatment of Mia Rinaldi. His actions in life influence Lissa and Mia throughout the first book.
Eric Dragomir, the dead father of Lissa and Andre, is fleshed out a bit. He grew depressed at realizing his family was dying out and had an affair with Las Vegas dancer Emily (Mastrano). They had an illegitimate daughter, Jill. He set her to be raised away from Court, financially supported Emily and her daughter, and ensured that his daughter would inherit a fortune upon reaching adulthood.
Frederick Dragomir, the father of Eric, is also introduced to the reader posthumously. He runs for king of the Moroi, passed all the required challenges, and quit at the last moment. He had learned that he had become a grandfather and decided to devote himself to his family.
Saint Vladimir and his dhampir guardian Shadow-kissed Anna are introduced through written accounts, dating to The Middle Ages. They are the second pair of spirit user and shadow-kissed companion introduced in the series. Rose studies their life to learn more about the nature of Lissa's and her own powers.
Warrior Cats has StarClan (the afterlife in the series), so cats that die usually aren't gone gone. Though they do eventually fade away. Yellowfang and Spottedleaf, for example, probably did more in the series dead than while alive.
Also, Redtail and Oakheart are surprisingly important, and things they did before dying are very important in the original series.
In Wilder Girls, this applies to Mr. Harker, Reese's father and the school groundskeeper. He is presumed to have died by the time action starts, 18 months after the school has been quarantined due to a disease called the Tox. Reese is hopeful he's still alive somewhere until she and Hetty encounter his monstrous, Tox-animated corpse.
. His attempts to learn more about the man by interviewing his children, which eventually leads to him witnessing the end of the world, caused by another of Dr. Hoenikker's creations.