The stories my family members tell of living in Saudi could fill a book or two (and numerous blogs like this one); several expats have published books about expat life there. One can summarize it this way: There are good reasons that engineering and construction firms would pay engineers double to work in Saudi. For most, though certainly not all, the experience is generally unpleasant, made bearable by the company of other miserable expats and third-country nationals one might encounter.
It's not the Saudi royal family's Sunni affiliation that leads them to such religious stringency, but the Wahhabi strain of Sunna that the royal family has embraced since before the kingdom was founded. Without understanding the precepts of Wahhabism, one cannot hope to understand the Sauds' relationship with the US, Israel, Iran, or their own people.
I managed to keep my Goodreads review (see below) rather brief, but you can read some more extensive reviews here. One can also check out World Beyond War's David Swanson's review on Counterpunch.
Here's what Medea Benjamin does quite well: She describes, without resorting to panicky hyperbole or histrionics, the history of the Kingdom of Saudi Arabia and its relationship with the United States since its formation in the 1930s. That history includes the al-Saud family's embrace of Wahhabism and imposing this interpretation of Sunn'i Islam not just on the kingdom's subjects, but also the millions of workers brought in from other countries for menial labor (who are frequently ripped off and abused) and any Westerner who ventures outside his or her corporate compound.
Here's what Medea Benjamin does in the introduction but not sufficiently, in my view: She takes some pains to caution readers not to conflate Wahhabism with mainstream Islam, or Saudi laws and customs with those of most Muslim nations. The Saudis are the exception, not the rule. Americans often think of Iran under the ayatollahs as the ultimate in repressed societies, but Iran looks downright permissive compared to the KSA. Benjamin should have placed a reminder at the head of every chapter that the nation ranked consistently the top violator of human rights is an important US ally, which says a lot about the US.
Beyond that, Kingdom of the Unjust is an excellent and necessary book. Benjamin began it before the US got involved in the KSA's military escapade in Yemen, the Kingdom's first act of international aggression (unless you count the Yom Kippur War of 1973). It helps to know which side the US has taken in that conflict (spoiler alert: it's the side with all the oil & all the money).