The JEDP hypothesis (the idea that the books of Moses were actually composed by four anonymous authors) is in pretty bad shape, but that it persists because no one can think of anything to replace it. Friedman in writing this book does not offer many specifics in defense of the theory, and in fact seems unaware that it is in terminal condition: That's hardly good policy, to treat a patient by ignoring his symptoms.
Nevertheless, there is nothing new here, as psychological mirror-reading is used to create explanations in support of JEDP. Why is Ephraim blessed over Manasseh? Because King Jeroboam was from the city of Ephraim...but doesn't that make Manasseh sort of superfluous? The ancient Israelites sure invented a more cumbersome history than they needed.
Admittedly, Friedman's tone is not that of a hostile critic, and he rightly supposes that later editors did indeed update the books of Moses to eliminate anachronistic information. JEDP, however, simply takes things too far, and its very artificial nature becomes all too evident when one finds so many exceptions to the given rules and has to explain them away with either the random actions of redactors or some obscure psychological or historical explanation without a shred of evidence.
For example, Friedman tries to support his thesis by suggesting that only the Levites were in bondage in Egypt, and that they somehow managed to gain enough influence to take over Jewish religious life to a significant degree. It is amazing how much history is created in the process of trying to explain away the history recorded in the Old Testament.
For all of the creative explaining that is done, Friedman's book is undone by an effort to be too breezy and uncritical in its assumptions that JEDP is set in stone more surely than the Ten Commandments.