The phrase this book
is a reference to the entirety of the Revelation Christ gave to John—a copy of which each of the seven churches would receive. The plagues
described in this book includes specifically the penalties upon disobedience as experienced in this life and listed in passages as Revelation 6:3–4, Revelation 8:6–9:21, and Revelation 16:1–21. It is comprehensive, as was already clear from the Old Testament (Leviticus 26:14–46; Deuteronomy 28:15–68 and especially Deuteronomy 29:20–21).
18 I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book,