Harlan Ellison
*The Last Dangerous Visions *
current status: Originally due in 1972. Ellison died in 2018. Some stories from the book have been published separately
David Gerrold
A Method For Madness and the rest of The War Against the Chtorr series
current status: The first four volumes were released in 1983, 1985, 1989, and 1993. There have been excerpts published since. Gerrold insists he is still working on this series.
Robert Jordan
The rest of The Wheel of Time series
current status: Jordan died in 2007. Some of the unfinished books in the series were finished by Brandon Sanderson. The main series is concluded but Jordan had planned on writing at least five other books in the same setting.
Donald Kingsbury
The Finger Pointing Solward, sequel to Courtship Rite (1982)
current status: Kingsbury says he has been revising it. Since 1982.
George R.R. Martin
The conclusion of A Song of Ice and Fire series.
current status: The first five volumes of the seven volume series were published between 1991 and 2011.
David R. Palmer
The final book in the To Halt Armageddon trilogy.
current status: The first book, Threshold, was published in 1985. It seemed like the series was dead. But then the second book, Special Education, was published this year. So who knows?
Melanie Rawn
The final book in the Exiles trilogy.
current status: The first two books were published in 1994 and 1997. But Rawn has moved on to other series and the third book, The Captal’s Tower, was never written.
C.S. Forester
Hornblower and the Crisis
current status: Forester died in 1966 so this one is looking increasingly unlikely. His outline was published and some authors have written a full version but there’s no official book.
George MacDonald Fraser
Flashman’s Civil War book
current status: Fraser talked about Flashman’s adventures during the American Civil War but he finally admitted he had no plans to write them out in a complete book. Then, to emphasize his point, he died in 2008.
Other examples are welcome.