One of the interesting things about the Bible is that it is both a single book and a collection of books. It has both a single author and multiple authors. It is both one story and yet multiple genres, stories and writings.
At any given moment, there is usually a push to treat it more clearly as one or the other. So, lots of people have made effort to ensure that we preach the Bible as one story – which it is – but then can so emphasise the oneness of the story and overarching author that it flattens the differences between the multiple authors. Others, by contrast, so emphasise the different authors and genres that they almost (or, sometimes, totally) ignore the fact that there is one storyline to about which the whole thing points in every part.
There are often different occasions to emphasise one thing or another. So, in preaching, I tend to emphasise the oneness of the story for believers reading the scriptures in light of the Christ to whom they point. Whether reading Old or New Testament narrative, poetry, prophecy or history the emphasis falls hard on the primary author (God) and the key to the storyline (Jesus Christ) and the reason for his coming and the occasion of any promises (the gospel). Whilst we are, of course, looking at the details of this particular book, we are concerned about them so far as the overarching storyline goes too.
But in evangelistic settings, though not overlooking that there is one overarching author (God) and one key storyline (the gospel of Jesus Christ), the emphasis more often falls upon the multiple authors, across different times from different places. This emphasis may come in response to somewhat facile questions about the “original” bible. It might come up in speaking with Muslims who are arguing, without irony, that their book written by one person in one place (despite its internal inconsistencies) is better evidence of divine authorship than a consistent story written by 40 different people over 1500 years or so, most of whom never met. It might come up in honest discussions concerning how and why we should trust the Bible. But in any of these cases, an emphasis on consistency among a significant number of people who wrote in their own styles, from their own cultures and times, but who all agree is often key.
Of course, even in those apologetic situations, the point remains that the multiple authors, with their different cultures, times and genres all agreeing together points to the fact that there is a single author inspiring the whole thing and a consistent storyline he is unfolding. But the emphasis for the sake of evidence and trustworthiness falls on the multiple authors. Four gospel writers who all share their own stories acting as separate witnesses rather than the whole book acting as a single entity. Likewise, when we are preaching, the single author with his one storyline emphasis does not mean we spend no time in the details or notices how this writer differs in style, tone, emphasis and other things to other writers. But the emphasis fall on the overall big picture.
In neither case do we ignore the one author/one story fact. In neither case do we overlook the multiple author/multiple genres/multiple angles fact either. But in different situations we may emphasise one thing over the other. There is but one author with a single storyline which is supported by the fact that multiple writers, speaking across different times and places, all consistently unfold the same story in different ways. We need to hold both these things together.
That is the answer to the question. Is the Bible one book or many? Ultimately, it is both. It is both one book with one author and one story and yet a collection of writings, from multiple authors, from different places, using different genres yet consistently agreeing with one another. It is both one book and many. It both has one author and many. It is both of these things that speak to its consistency and act as strong evidence it is, indeed, divine revelation.