L. Frank Baum
The Oz series
Ruth Plumly Thompson wrote 21 additional books after Baum’s 1919 death.
The Bourne series
Ludlum wrote three Bourne books. Twice that many have come out since he died.
The Wheel of Time series
Brandon Sanderson used Jordan’s notes and outlines to continue his massive fantasy saga.
The Dune series
Thirteen years after his death, Herbert’s son began co-writing prequels.
Robert B. Parker
The Spenser and Jesse Stone series
Parker died in 2010, but his iconic characters will live on.