So ... what, exactly is the book of Revelation, anyway?
That may seem like a simple question, with a simple answer: It's a revelation. But, of course, the book of Revelation is not the revelation itself. It is a recording, a writing, of the revelation, which John heard and saw. And that has several implications for us, the readers.
First, this genre of writing is often simply called apocalypse, which is the Greek word for 'revelation' taken from the first word in the book of Revelation. Remember, none of our biblical books came with titles; most of their titles were added later reflecting the first word (e.g. Genesis, Revelation), the assumed author (Matthew, Mark, Luke), the primary characters (Samuel, Kings) or some kind of description (Deuteronomy [the reiterated law], Psalms, Acts of the Apostles).
Apocalypse, as a genre of writing, is very hard to nail down- as it ought to be. John is writing about things he has seen and heard. But, what John has seen and heard are things that are too wonderful for literal description. So, the continuity between the revelation itself and the book of Revelation is met with a discontinuity: he is writing to faithfully describe things which are indescribable. Therefore, what we encounter in the book of Revelation is an attempt to use words, language, descriptions, etc. that point beyond themselves to a reality that is greater than any words, language, descriptions, etc. can possibly proclaim.
In short, apocalyptic writings are necessarily written in symbolic language, because literal language is impossible for this task. So, they reveal, but- in a way- they also conceal, because the words themselves are taken from language that we often use literally and are put to non-literal use.
Second, even if John were somehow able to fully describe his vision, there is an inability on our part to comprehend the kind of wonder-filled glory- either beautiful or tragic- that is revealed to John. The symbolic language of apocalyptic writings is as much a sign of our finitude (the readers) as it is sign of John's finitude (the seer/writer). The language of the 139th Psalm is appropriate any time we read descriptions of God's glory: "Such knowledge is too wonderful for me; it is so high that I cannot attain it."
I think my greatest fear of even broaching the book of Revelation (and other apocalyptic writings) is that too many of us just blunder right into it with a sense of confidence that is undeserved. Whether we are biblical scholars who speak from the perspective of informed scientific critical skills, a preacher declaring that this or that verse is exactly what this or that nation needs to hear, or a deconstructionist pulling and twisting the literary structure at will- all of us ought to approach any work of apocalypse with a modicum of fear and trembling.
It may be that the very word, verse, concept, or idea that we are addressing is simply too deep for us to understand- much less to explain! That does not mean that we have to shut up. It only means that when we talk, we have to do so with an appropriate sense of humility.
So, back to the originating question: WHAT IS the book of Revelation? It is a book, a writing, using the limited and finite human means of language to describe what is ultimately indescribable. The revelation itself was probably too much for John to handle in the first place. Then, that incomprehensible revelation became what John witnessed, which was then written down.
Do we have a choice but to be humble about this reading?