The scene starts on page 65 of the 1996 paperback Black Fleet Crisis 1: Before the Storm. Han goes to visit Luke in Vader's Magical Shrinking Happy Family Castle (not really called that, but close enough) and Luke explains why Yoda and Ben were hermits and how they were so powerful in the Force that they couldn't take out Vader or Palpatine because it would be too close to the Dark Side to exert their powerful will on the Universe that way.
Interesting stuff.