The Revelation of Jesus Christ, which God gave Him to show His servants– things which must shortly take place. And He sent and signified it by His angel to His servant John, who bore witness to the word of God, and to the testimony of Jesus Christ, to all things that he saw. (Rev 1:1-2 NKJ)
What Kind of Book is it?
The opening verses of Revelation suggest that it’s not just one style but three.
First—Revelation of Jesus Christ. The word `revelation’ or `apocalypse’ (apokalypsis) suggests that the book belongs to the style of ancient Jewish and Christian literature, which modern scholars call apocalypses. John used this style of writing to hide what he had to say from the Roman government.
Second, Then he said to me, “These words are faithful and true.” And the Lord God of the holy prophets sent His angel to show His servants the things which must shortly take place. “Behold, I am coming quickly! Blessed is he who keeps the words of the prophecy of this book.” (Rev 22:6-7 NKJ)
For I testify to everyone who hears the words of the prophecy of this book: If anyone adds to these things, God will add to him the plagues that are written in this book; (Rev 22:18 NKJ)
So prophecy in the New Testament is more a proclamation, and should not be confused with the dispensation of Old Testament prophets. In chapter 22:18, John is so sure that he has a word from God that he didn’t want anyone adding to it or taking away.
Last, it’s a letter. This would have been set to one of the seven churches under John’s care and past on once it had been read in the congregation, often referred to as a circular letter.