The Phantom Menace, starting the Canon prequels, was released in 1999.
Back then, I came across an old SF magazine that described the Extended Universe sequels to the Original Trilogy. Prequels were not mentioned, the article was incomplete (to be continued in the next issue of the magazine).
I wonder if there were EU prequel novels or comic books before the Canon prequels came into existence. How much do they contradict the canon?
PS. I am aware of the non-canon alternative prequel trilogy by David Gremillion (Awakening - Horizons - Hunted), but this was written after the canon prequels, as an alternative, and is not officially canon at all (not EU, considered fan fiction), so this does not count.