ToscheStation said:
The way Lucas described “book three” back in Dec 1975 says otherwise:“In the third book I want the story to be just about the soap opera of the Skywalker family, which ends with the destruction of the Empire.”
Source?
The Making of Star Wars (JW Rinzler, 2007). It was during a story meeting with Kurtz and writer Alan Dean Foster. In it, he also said that at the end of the second book, “we find out WHO Darth Vader is”. (emphasis mine)