Is the novel Darth Plagueis by James Luceno considered “canon” in the Star Wars universe?
Although an excellent book widely considered a fan favorite, Darth Plagueis now falls under the banner of “Legends” — tie-in fiction from the vast Star Wars “Expanded Universe” conceived before Disney bought Lucasfilm and essentially hit the reset button on all those novels, comics and games.
Here is the “new” Star Wars literary canon, as defined by the “Story Group” at Lucasfilm charged with producing official fiction related to the movies and television series set in that Galaxy Far, Far Away.
The character of Darth Plagueis is canon, however, as he was mentioned on screen by Chancellor Palpatine in Star Wars: Episode III — Revenge of the Sith. This means that the character could conceivably re-enter the core Saga but with a new interpretation — subtly or even wildly different — in the future. There is precedent for this already.