Lewis is cautious with respect to his final conclusions about the origin of "Yahweh," that is, where and how a god named Yahweh came to be Israel's God. And yet, this most interesting part of Lewis' book (based on archeology, and texts both Biblical and from surrounding cultures) also suggests that Israel's adoption of Yahweh was a slow, politically fraught process. And that what we have in today's Hebrew Scriptures represents a late reordering and rewriting of ancient traditions to better serve the emerging Judaism, both official and familial, of that time. Lewis' treatment of Yahweh's history should be read with Adler's "The Origins of Judaism," and van Oorschot's "The Origin of Yahwism." There are two or three competing theories out there in the latest scholarship, but they all agree that the official adoption of Yahwism was a long slow process whose progress is sometimes difficult to track, in large measure because of the programatic effort of priests and scribes to make that adoption happen (for which see Karel Vander Toorn's "Scribal Culture and the Making of the Hebrew Bible."
I found this to be the most interesting aspect of this book.
Lewis spends most of his time delving into the many sides to Yahweh's character, and this part of the book is best used, I think (and as Lewis suggests) as a something to dip into if you are interested in some specific aspect of who Yahweh is in scripture and what he was for the Israelites.
Highly recommended. Not an easy or brief read, but a very satisfying one.