Imam Ali's letter is just astonishing. The author did some nice job providing some commentary and analysis but he could do a better job no doubt. I have to say that the book warrants 2.5 stars but the 3rd star is because of the fact of the impressively powerful statements of Imam Ali, making it a must book for people with interests in understanding the political and economic system/framework from an Islamic perspective.
You can, of course, read Imam's letter plainly but some commentary and analysis helps (even if it is not the greatest), hence the recommendation.