Champion Challenge

Champion Challenge was a limited duration events that featured a series of 15 and 20 stamina battles against 'legends'. Those battles are relatively easy, and had a decent chance to drop said legends books. Almost all the battles could be done with double Neptune/EXPPerona team with little difficulty, or Buggy+Neptune/EXPPerona.

For details on who you could fight and what books would drop, see Reddit post here.