I fear cynicism has crept into my thinking as the years go by to the point I don't think "canon" means very much (since it means whatever they tell us it means). I know what I like and see as more authentic, but I don't know that it matters since I'm yet another person saying X is canon. All that blathering aside: ANH, ESB, and ROTJ constitute my canon. Other stuff is interesting, good, bad, and ugly.

Of note is the prologue to the ANH novel:

Once secure in office he declared himself Emperor, shutting himself away from the populace. Soon he was controlled by the very assistants and boot-lickers he had appointed to high office, and the cries of the people for justice did not reach his ears.

"Having exterminated through treachery and deception the Jedi Knights, guardians of justice in the galaxy, the Imperial governors and bureaucrats prepared to institute a reign of terror among the disheartened worlds of the galaxy. Many used the imperial forces and the name of the increasingly isolated Emperor to further their own personal ambitions.