The book interprets events in Mahabharata while trying to draw parallels from the contemporary world.
Following are the key takeaways based on my understanding of the book. I am assuming some familiarity with the story of Mahabharata.

Difficulty of being good

Goals of human life
Mahabharata mentions three goals of human life – artha (money), kama(desire) and dharma(righteousness).
Note:moksha (salvation) was added in post Mahabharata period.

Subtle
One “subtlety” is that when all three goals of life cannot be attained simultaneously and the choice has to be made between artha+kama and dharma.
Another “subtlety” is of dharma, it is difficult to put people/choices in rigid compartments of good and bad, making right decisions is, therefore, subtle.
Therefore, Mahabharata does not compartmentalize people into good and bad, rather explains different (subtle) viewpoints on every crucial decision of its characters.

Dharma
Dharma is subtle, many times, we are forced to take wrong means for right means(and ends us like Arjuna in hell) or reaches wrong ends following right means(like Yudhishthira in exile).
The distinction is made between two form of dharmas – sva-dharma (eg. fighting a war for warrior) and sadharana-dharma(eg. non-violence, remorse and compassion). In real life, these dharmas occasionally contradict each other.
Mahabharata rejects envy-filled Duryodhana as well as violence-avoiding (pre-exile) Yudhishthira, it praises a pragmatic “reciprocal altruistic” Yudhishthira (post-exile).
Mahabharata does not approve of non-reaction to injustice [“showing the other cheek” philosophy] for that signals the evil person that wrong act pays.
The text does not treat sva-dharma or sadharana-dharma superior to each other but suggests a middle path.
sva-dharma asks for a man to carry friendly but reciprocal altruism, that is, a cautious face towards the world, while sadharana dharma asks for a non-reaction even to injustice(anyaya)
Krishna teaches Nishkaama Karma [“be intent on the action and not on the fruits of the action”] to Arjuna which is the central idea of Karma Yoga. Clearly, the emphasis is on “sva-dharma” over “sadharana dharma” [not only the preference of sva-dharma is questionable but also the identifying sva-dharma is a mystery in itself].

Human Emotions
Duryodhana’s envy for the Pandva’s Indraprastha and Yudhishthira’s greed, or addiction, to a game of dice and win more territory was the main cause of war.
The envy can result in healthy competitiveness raising oneself up by making the envious work harder, for rival’s possessions, or eternal sickness to pull the rival down.
The envy and greed are both presents even in the modern day world, the failure of communism was caused by envy among citizens while the capitalist system is stranded by greed.The aim of society should not be to suppress these feelings among its citizens but to channelize them productively. Envy is the vice of socialism, Greed is the evil of capitalism.

Karna always had a status anxiety, for being brought up in a family of low caste charioteers. He was aware of the fact that he is kshatriya by blood. Interestingly, the author points out that less feudal societies like the USA suffer more status anxiety than more feudal European nations. The existence of opportunity causes more anxiety. Eklavya, being born in tribe felt less anxiety than Karna, even when he was denied Kshatriya training. (ashishb’s note: when Dronacharya asks Eklavya’s thumb as a guru-dakshina, author calls it “caste-discrimination”, I see this as an example of ego(ahamkara) and not caste-discrimination, since, Dronacharya wanted to see Arjuna as the best archer and thus, would have asked for same guru-dakshina irrespective of Eklavya’s caste)

Draupadi’s question “what’s the dharma of the king” implies accountability on the part of administration on what happens in their jurisdiction irrespective of who does it.

If God exists, why is there evil in the world?
God exists but is not all-powerful, its the law of karma which is all-powerful. Human beings are free to “act” but they cannot escape from the consequences(niyati) of past actions(karma) and strokes of furtune(daiva)
[ashishb’s note: In engineering parlance, we have a trade-off here, either human can act freely and some of them will cause evil or they could  live like slaves of God and thus, no evil, the law of karma suggests former]

Retributive justice vs Forgiveness (justice vs peace)
While nation with stronger (legal) laws can prefer justice over peace, the newly created or war-torn nations with weaker legal laws should prefer to restore peace and harmony in society rather than emphasizing justice. After the end of Apartheid, South Africa preferred peace and succeeded while Argentina preferred justice, tried prosecution in 1980 of dirty war which was sponsored by the previous government and failed.

The yaksha prashnas (questions with Yudhishthira’s answers)
1. Who is happy?
One who cooks vegetable at home has no debts and is not in exile.

2. What is extraordinary?
One sees people dying every day and still thinks he will be live forever.

3. What is the news?
Time cooks human beings.

4. What is the highest dharma in the world?
anrishamsya (compassion)

The famous [agnostic] Nasadiya verse from Rigveda
There was neither existence nor existence then … There was neither death nor immortality then. There was no distinguishing sign of night nor of the day … Who really knows? … The gods came afterward with the creation of the universe. Who then knows whence it has arisen?