Canon (In this context) means Tv or (sometimes) Movies.
Books and comics don't count.
Some of the books made while TV shows were running were awful and quickly became ridiculous before the tv heads off to unexpected places.
Sometimes a writer will say THIS IS CANON! about a book they just wrote, but Jeri Taylor was just being an ass, venting all the girl bollocks the boys club wouldn't let her plaster all over Voyager.
The Star Wars Universe, for a while there, everything was supposed to be the same continuity., but they have to have given up the ghost on that now what with Clone wars into it's 4th season.