The Oz books: the official series consists of 40 books, of which nearly two-thirds were written by other authors (including Baum's grandson) after L. Frank Baum's death.
Peter Pan had sequels produced after JM Barrie's death in 1937, at least one of which, Peter Pan in Scarlet, was actually commissioned by the children's charity to whom he bequeathed the rights.
Specific still-under-copyright examples
Robert E. Howard's Conan the Barbarian is a prime example. Howard committed suicide at only thirty years old and left a number of stories unfinished, leading first to posthumous collaborations and then later to full-blown original short stories and novels by other authors and an ongoing line of comic books.
V. C. Andrews' generational sagas about the Dollanganger and Casteel families were continued after her death by a ghostwriter, who then went on to write several more series along similar lines that were published under her name.
Stan and Jan Berenstain, the original creators of the The Berenstain Bears series of books, have both passed away since the book series' creation, Stan in 2005 and Jan in 2012. They'd handed the series over to their son, Mike in 2002, and after his father's death, he'd written multiple books with his mother until her passing. Nowadays, Mike continues to write The Berenstain Bears, which has taken a major religious turn since he assumed full ownership in 2012.
The final three books of The Wheel of Time series were completed by Brandon Sanderson after Robert Jordan's death in 2007. After a terminal diagnosis in 2005, Jordan began to prepare notes so that another author could finish the series, and Sanderson was chosen for the job by Harriet McDougal, Jordan's widow.
James Bond. Original author Ian Fleming died in 1964 while the third movie Goldfinger was in production, and more than twenty new films have come out since then. Not to mention subsequent books and video games (some with original plots).
The Tertiary, Quandary, and Quintessential Phases of the radio series (based on the later books) also came after his death (though Adams had planned for radio versions of them, even recording himself playing Agrajag). And Another Thing eventually saw a radio adaptation of its own as the Hexagonal Phase.
The Bourne Series has outlived Robert Ludlum. Ludlum wrote the first three novels, and died long before the film adaptations were released.
The Little House books. Laura Ingalls Wilder's will stipulated that her daughter, Rose Wilder Lane, would hold the rights until her death; after which, the rights were supposed to go to a local library in Missouri. Lane's heirs managed to get the rights back, and HarperCollins has resurrected the franchise with sequels, spinoffs and prequels.
Dune. Since Frank Herbert's death, Brian Herbert (Frank's son) and Kevin J. Anderson have written a number of prequels and sequels.
The Boxcar Children series had only 19 books written by its original creator, Gertrude Chandler Warner. Then, over a decade after her death, Albert Whitman of Albert Whitman & Company resurrected the series, producing over 140 more books due to reinterest (including a kid-friendly cookbook). After Warner stopped writing them, continuity went right out the window and quality noticeably dropped.
The Godfather had two sequel books (The Godfather Returns and The Godfather's Revenge) and a prequel (The Family Corleone) that were written and published by other authors after the passing of original author Mario Puzo.
The St Clare's series by Enid Blyton. The St. Clare's series consisted of six novels that spanned the six years set in the school. Three books covered the first year, one for the second, one for the fourth, and one for the fifth. In 2000, Pamela Cox wrote two more St Clare's books, one set in the third year and one in the sixth; at least one was just a pastiche of previous plots with wildly out-of-character moments and anachronistic phrases.
Likewise, Pamela Cox wrote extra books for Blyton's Malory Towers series, even though the series was considered finished when Darrell Rivers, the main character, left the school in-story. The new books follow her younger sister, Felicity, and are again an Anachronism Stew, filled with modern phrases that would not have been used in The '50s when the story was written.
Also with The Secret Seven: French writer Evelyne Lallemand wrote about a dozen further novels during the '70s and '80s, most of which were later translated into English and published with Enid Blyton's signature on the cover.
While Blyton completed the Noddy books in 1963. Beginning in the early '70s, newer stories and books starring the titular character were published after Blyton's death in 1968.
After co-writing seven of Robert Asprin's Myth Adventures novels, Jodie Lynn Nye continued the series after his death, publishing her first solo volume in 2012.
Even after Rex Stout's death, and what seemed to be Stout's final story in the series, Nero Wolfe picked up another author, Robert Goldsborough, who called it quits after writing seven additional stories.
Fritz Leiber's Fafhrd and the Gray Mouser series has seen one new novel written by a different author after Leiber's death, although the new book is set between two of the original ones.
For almost forty years after J. R. R. Tolkien's death, his son Christopher Tolkien was kept busy editing and putting out his posthumous works, with profuse annotations and tweaks. Taken a step further with new releases even after Christopher's death, with new editors picking up where he left off.
Michael Kurland, a friend of Randall Garrett, wrote two novels about the Alternate History detective Lord Darcy and his sorcerous assistant after their creator's death.
The Mr. Men and Little Misses books were written by Roger Hargreaves until his death in 1988. His son Adam took up writing them instead, but Roger's name continues to appear on all the covers, even the new ones.
Both Nancy Drew and The Hardy Boys have outlived their most important creators. Edward Stratemeyer (who created the concept), Mildred Wirt Benson (Nancy's most prominent ghostwriter), Leslie McFarlane (the Hardys' most prominent ghostwriter), and Harriet Stratemeyer Adams (the editor whose borderline Orwellian Retcons of the series kept them from fading away) have all passed away, but new books are still being written.
The Railway Series by the Rev. Wilbert Awdry is still being continued even after his death in 1997, the torch having been passed to his son Christopher Awdry. The quality remains good, as Christopher (whom the stories were originally created for) has the same innate grasp of the series as his father and is dedicated to making sure it remains high-quality children's literature.
Tom Clancy died in 2013, shortly before his last book came out. Other authors started publishing sequels to his final work less than a year after his death.
Alfred Szklarski died in 1992, and apparently, so did Tomek Wilmowski Series. His friend, Adam Zelga, wrote Tomek in the Tombs of the Pharaohs, published in 1994, supposedly based on Szklarski's notes. Then, in 2021, after twenty seven years, Maciej Dudziak published Tomek in Alaska, also citing notes left by Szklarski. In both cases, the fandom shuns the books as a blatant copyright grab by a new publisher, who simply needs to justify holding the copyright over the whole series with something (the books actually written by Szklarski were printed by a different publishing house, headed by Szklarski himself, and went out of business after his death).
The Amber Brown books were released from 1994-2004 by Paula Danziger. In 2012, the series was revived by Bruce Coville and Elizabeth Levy, whom Danziger described as her two best friends, with permission from the family. Reviewers generally agreed that the quality was so good that readers would not know the new books were not written by Danziger if they didn't know she was dead and if not for Coville and Levy's names on the covers.
Since Steig Larsson passed away before the first book of The Millennium Trilogy was published, he was only able to write three books for his planned decalogy. But in 2015, the publisher decided to hire author David Lagercrantz to write more books in the series, which will be entirely his own invention without any inspiration from Larsson's plans.
Years after Richard Scarry's fatal heart attack in 1994, the BusyTown series still gets published with new books every few years. Even getting a second animated series by Cookie Jar Entertainment in the late 2000s, but was very short-lived unlike The Busy World Of Richard Scarry from the late 80s and early 90s.
Robert B. Parker had several series in circulation (including Spenser, Sunny Randall, and Jesse Stone) when he died at his desk in 2012. Some of those series are being continued, with a set author for each one (which would at least help keep the tone consistent).
Amelia Bedelia still receives new stories after Peggy Parish's death, courtesy of Herman Parish, Peggy's nephew.
Noboru Yamaguchi, author of The Familiar of Zero, died of cancer in 2013, with 20th light novel volume of the series released 2 years prior but with some material to release the following volume ready, the series was nearing its climax and was a constant hit in sales; Noboru had rough notes and manuscripts of how the series would end, with this the publisher Media Factory decided to pick someone capable of finishing writing the series in his place, 2 more books to end the series at the 22nd volume were scheduled, in February 2016 the 21st volume finally reached the stores, the new author was being kept in secrecy, Noboru Yamaguchi being credited as the sole writer as tribute. The new author was since been revealed as Yū Shimizu of Bladedance of Elementalers.
The Netflix series of Llama Llama started production during Anna Dewdney's final stages of breast cancer. It currently lives on as an animated series on Netflix, merchandise, and occasional stage shows. Some of the original Llama Llama books that were unpublished ("Llama Llama Loves To Read" and "Llama Llama Easter Egg") managed to get released two years after her passing. Future Llama Llama books by Dewdney (such as "Llama Llama ABC") would also get released posthumously using Dewdney's manuscripts and illustrated by JT Morrow.
Despite Don Freeman (author of Corduroy) passing away in 1978 not long after the release of the sequel book A Pocket For Corduroy. Beginning in the 1990s, newer book starring Corduroy were released under a different illustrator and a different universe (Corduroy lives in a world where toys are alive instead of a real world setting) with the notably absence of Lisa and other human characters. However, two official follows ups (Corduroy Lost And Found and Corduroy Takes A Bow) and a prequel book (A Christmas Wish For Corduroy) are a direct continuation of the original Freeman books.
Jill Paton Walsh completed the unfinished Lord Peter Wimsey novel Thrones, Dominions in 1998, forty years after Dorothy L. Sayers' death. She went on to write three original novels.
Downplayed with the Discworld: Shortly after Terry Pratchett died there were reports that Rhianna Pratchett would be continuing the series, until she clarified that she had no intention of doing anything of the sort, seeing her role as protecting her father's legacy, including from herself. However, the supplementary "reference-style" books are a bit different, as many of them were at least partly witten by other people even when Sir Terry was alive, and so Tiffany Aching's Guide to Being a Witch by Rhianna Pratchett and Gabrielle Kent was announced in 2023.
