Here is another quote from John Sailhamer’s The Meaning of the Pentateuch. Here he discusses how to identify the big idea of the Pentateuch (p. 151).
“How then, do we find the big idea of a large text such as the Pentateuch? How do we formulate a big idea to help us make sense of the Pentateuch? The answer is surprisingly simple. We find the big idea by reading and rereading the text in its parts and as a whole. As we read the text, we begin to formulate a sense of what the text is about (subject matter), and where the author is going with it (compositional strategy). Most of the time, we do this unconsciously, but as we reread the text, we should also do this consciously and intentionally. We should look for where the author is taking us in the Pentateuch. What kind of topics and events are important to him? What is he writing about? What is he saying? Those questions help us get a sense of what the Pentateuch is about.”