From a fun thread on r/StarWarsEU:
^ a screenshot of the 1997 ‘The Star Wars Trilogy Scrapbook: The Galactic Empire’
https://starwars.fandom.com/wiki/The_Star_Wars_Trilogy_Scrapbook:_The_Galactic_Empire
There was also mention of the 1993 X-Wing game, and “The Farlander Papers” mission & companion book that came with it:
https://starwars.fandom.com/wiki/The_Farlander_Papers
Mon Mothma speaks of the “decades of peace” between the end of the Clone Wars and the rise of the Empire, with Palpatine being elected President during the war, but not declaring himself Emperor and exterminating the Jedi, until much later.
Some pages relevant to the “pre-Prequels” story at the time:
All 98 pages of it can be seen here: https://archive.org/details/pdfy-52fQeoxh9CohzbEk/page/n67/mode/2up
Some pages from Mon Mothma’s “A Call To Reason” manifesto-type document used to try and attract people to the Rebellion (and part of The Farlander Papers):
Edit: there is more on this, and the pre-PT era lore in general, in this new thread:
Pre-PT era lore | an OT & EU scrapbook resource | additional info & sources welcome