I'm reading a handful of Vatican II documents right now (as the Vox Ecclesiae page attests) -- the only one I'd read in its entirety had been Sacrosanctum Concilium, but now I'm reading a group that have to do with religious liberty, ecumenism, and the identity of the One Church of Christ as the ("Roman") Catholic Church.
But I'm also going to revisit Sacrosanctum Concilium eventually too, because I'll be looking at its call for liturgical reform; in the mean time, check out this article from Sacred Music's Winter issue in 1987. It's by Msgr. Richard Schuler, and it's called "Participation". It analyzes the meaning of "active participation" (actuosa participatio) as presented not only in Sacrosanctum Concilium, but in no less than 5 pre-Vatican II Papal or Magisterial documents!