Nastik is an Indian take on Eastern religions as a whole but it trades the indigenous roots for the global perspective. It may be possible that the "Eastern" religion in practice is not well understood by the author either.
Let's start with the great: the book offers a good theological opposition to the Abrahamic religions; all of which are of Jewish origin. This "monopolized monotheism" and assumption that the Universal God is also one's personal God has been the most catastrophic assumption for humans.
The book even goes after Buddha's belief in the Varna system, although this may be reflective of his belief before sanyas.
The author, although, makes many errors in this itself. He confuses atheism, which is a belief, with Mimansa, which is a philosophy. This is comparing apples to oranges. The Mimansa is arguing philosophically that even if there is no greater Brahman, there should not be a lack of practice with a personal deity.
The book presents the side of the 4 horseman of atheism in full, for a whole chapter, without any comment. On the other hand, he presents the Indic side with minor critiques, and doesn't even go into what the scriptures say.
The book doesn't refer to the scriptures of the Hindu faith beyond a point either. It doesn't even refer to the Charvak school scriptures, or make the effort to have the manuscripts translated.from someone else.
The assumption that Japanese society is not plagued with modern caste problems is also a big assumption. While Japan may have mitigated most of their problems of their system through prosperity and freedom at the bottom level, the legal protection to the Eda community was not given until the 2010s afaik.
The book is unable to critically analyse Indic scriptures either. While varna-jati-kul based discrimination has no place in modern society, the book even mentions the scriptures that say it was due to diseases. But it doesn't go into the functional aspects of the system from back then, only the dysfunction of it (or at least it doesn't go into positive functions deeply enough). The Manusmriti can be reinterpreted to mean "we stay away from those with diseases" and "kingdoms run by degenerates" (like the USA). But maybe that's the conversation of another generation, one which has read more scriptures.
Besides, one's jati/kul identity is what roots them to the ground of this country (this bhumi). It is not blind visit to temples. It only produces blind faith and no improvement in one's life. One could maybe detach this jati/kul identity from the Vedas, Upanishads, Dharmshastras, etc. This is where Jatitva comes in, and it promotes the practice of regional butchering techniques, local meat and plants, dishes, our tribal identity (Agarwalrana, Valmiki samaj, etc). These achievements towards making dharma could be celebrated without practicing dharma. And the Vedas should be seen in a new light, under this method. To simply focus on the dysfunction, whether is it community temples or other institutions, is lost.
This caste practice is also why pluralism between Buddhism and Shinto exists. Many sects like Nichiren Daishonin emerged actively opposing the Shinto priestly class. Social reform has led to Japan's indigenous religion being the least practiced. Their texts don't hold precedence over the people either. The book ignores this yet again in an attempt to homogenize the Hindu society.
Whether it is secularism of the old or hindutva of the new, when the conversation shifts from coalition to homogenization it repulses the people and opposition forments. We saw many regional movements from Maharashtra, Assam, Punjab, UP, Karnataka, Tamil Nadu and others when it came to opposing Indira Gandhi. Each was regionally chauvinistic and this led to the Ram Janambhumi movement afterwards. Now, Hindutva is doing the same homogenization which will have many regional-atheist revolts. The only certainty is that Muslims, a homogenizing force themselves, will not be a part of this conversation, being the foreign faith in practice.
The book only focuses on the "plurality" aspect of Eastern religions, but it doesn't go deep into the question of identity either. It is because of the lack of identity that otherwise comes with Jewish religions, that Eastern religions are more pluralistic. A Jew/Christian/Muslim is born one. An Eastern religion person becomes who they are through their practice. That is also at the root of "multiple sampradayas in the same family"; no one person practiced two sampradayas at once. This is what many raitacharyas get wrong on Indic religions, or their rhetoric doesn't reflect it.
The author could have put their own beliefs aside and tried to see this side of the world for what it is. Eastern religions focus on non-violence and non-meat (like Cao Dai in Vietnam, etc), they focus on a non-personified larger entity, they are about the communication is done through a deity between these two. They also, almost all, worship nature. But this also does not apply to all of them. There is more than simply western constructs of secularism and pluralism that hold us together. That is still not left explored enough.
Overall, the book is not the best at making a comprehensive argument on Eastern religions. It does, however, provide a political toolkit to bring "pagan unity" across the globe. But it is too nascent, and the book only does this latently, ie by mistake.
So it may be a good addition to the growing pool of "Indic" literature, but I doubt the longevity of this text in the long run. It may inspire many at best.
The book is criminally underrated in leftist circles, which is sad. It is where the book belongs, and it should be circulated in those circles more. Because even if every Bharatiya reads it, only one side will adopt it.
The book isn't bad, but it is overreliant on twitter thinkbros. It has started a political conversation, but let's see how it evolves and whether the entire country is able to take part in it.