Book Review: Understanding Genesis

Understanding Genesis by prof Nahum Sarna is an interesting read I recently completed.  While not an Orthodox book by any stretch of the imagination, it is a fascinating and in-depth look at the first book of the Torah from the position of an interested and sympathetic academic, one more interested in trying to understand the message the stories of our ancestors teach us than in trying to tear down the text in an attempt to delegitimize it.
The book generally follows the narrative of Genesis starting with the creation of the universe and ending with Yaakov Avinu and his sons arriving in Egypt.  Most of the chapters corresponded to a particular narrative and hence parsha but some are short enough to encompass only part of one. 
In each chapter the narrative is summarized and then analyzed for moral lessons.  The text itself is also compared to contemporary non-Jewish sources to look for common features and literary styles.  For example, Sarna contrasts the Babylonian/Akkadian version of creation with ours, showing how the non-Jewish version is essentially just a mythical story with nothing deeper to recommend it while moral lessons abound in the Biblical text, clearly the intent of the Author. 
In the chapter on the Mabul, for another example, Sarna again contrasts various flood stories from the region with ours and again shows that what we are supposed to take away from the narrative is not the technical details of what happened but the moral lessons that occur to Noach and other characters.  This is obviously a repeated theme throughout the book: the Torah is not a history text or dry religious tract but a book of moral instruction even when analyzed through a secular lens.
In fact, the one limitation of the book is the obvious: Sarna concludes (as any academic must) that the Torah itself is a multi-author work although many of the proofs he brings are incredibly weak or full of supposition and conjecture.  Where he differs from other academics is in showing that the literary style of Genesis, the various legal phrases in the narratives and the descriptions of life in the time of our Avos are authentic products of the time, not later invention from the time of Ezra HaSofer or later.  One example is Yaakov Avinu's final argument with Lavan in which Yaakov pretty much quotes his rights from the dominant code of Hammurabi which was the law of the land at the time, something that would have been unknown to another author centuries later.
In fact, that was the one frustration I felt when finishing the book.  According to Sarna, the narratives in Genesis can be ascribed to the time period they claim to be from but is a multi-author book.  According to Umberto Cassuto, the Torah is a unitary text but the product of a later human author.  How annoying.  Why can't they each reach the obvious conclusion that each of their analyses reach together: that the text is a unitary entity written at the time it claims to have been?