God speaks to His people through the Scriptures, as Catholics we believe the Old and New Testament to be inspired by the Holy Spirit, and so able to be called "the Word of God."
The second question I don't really know. Some of the Prophets pretty clearly talked to God and relayed what He said, others were writing down things that they may not have realized was inspired at the time.
Jesus is the Head of the Church, the Apostles are the first Bishops (and our Bishops are their successors), and the disciples are people who follow Christ.
The last Prophet was John the Baptist, because prophecy points towards Christ, and after John came Christ Himself, so there can be no more prophecy. (This is why, technically, the prophecies in the New Testament are different from those in the Old; the new ones are pointing towards Christ's Crucifixion or his Second Coming.)
I've heard this is a good book, and I've also enjoyed The Public Life of Our Lord Jesus Christ: An Interpretation which really opened the scriptures for me.