Star Wars opens talking about the old republic and how the emperor came to power. It mentions the journal of the Whills. But there isn’t really all that much in expanded material beyond Luke and his friends on Tatooine.
Empire i i think might have an extended scene but its pretty much a standard novelization of the screenplay.
ROTJ has an extended chat with Ben and Luke. That is now completely non canon because Ben and Owen are no longer brothers in canon.
Phantom Menace has deleted scenes.
Attack of the Clones has deleted scenes.
Revenge of the Sith is written from Anakin’s POV. Its the movie but better.
The Disney movies don’t really have much more material beyond the movie , Force Awakens has a scene of Leia in the New Republic and her aide the one who gets vaporized later when Starkiller base fires.
Last Jedi has more Luke and Leia and Han Solo’s funeral. ROS is pretty much the film except for the reveal that Palpatine is not only Rey’s grandfather but father.
The most expanded versions of Star Wars and Empire are actually the Radio Dramas by Brian Daley.