Rarely has a book so admirably met so compelling a need. Samuel provides a sweeping survey analyzing an incredible diversity of religious traditions, beliefs and practices over a great swath of time, and in so doing has produced the first inclusive general history of yogic and tantric traditions of India that I have yet seen, and I've been looking for a very long time.
Samuel traces the evolution of the two dominant idioms of contemplative practice in India from their origins in the mid-first millennium BCE to around 1200 CE. He traces the evolution of Hindu, Buddhist and Jain meditation practice over that span of time, contextualizing them in terms of their social idioms.
The reader will come away with a strong account of the evolution of solitary meditation schools out of the preceding Vedic culture, the bifurcation of spiritual practice into counter-posed communities of households and monastic communities, and the relationship between various schools of practice to social elites and the general population at large.
I benefited most from the terrific second half of the book which focuses on the history of tantra. Having reviewed an enormous literature on the subject, Samuel provides the first coherent and systematic account that I've seen of the entire phenomenon in all of its principle forms. He gives particular attention to the Saivite and Buddhist forms, but I came away with a deeply-enriched understanding of the whole picture, from the early days of cremation-ground practices and wild goddesses of the Deccan to the elaborate ritual forms encoded in the Kalachakra Tantra.
Anyone who has made a serious attempt to come to terms with the bewildering diversity of beliefs amalgamated under the label of tantra will find an invaluable guide in this wonderful book, which performs a feat that I might have previously judged impossible, giving a coherent account for how such radically disparate practices fell together.
The focus of Samuel's book is historical and social, not philosophical or soteriological, but I would urge anyone with an active personal interest in the material to read this book, because there are key aspects of tantra that literally cannot be understood without looking at the evolution of the body of beliefs.
What do mandala visualizations have to do with sexual yoga? What is the difference between tantric and non-tantric scriptures such as the Buddhist sutras, and why is there so much overlap between then? How do the Saivite and Visnavite tantric forms relate to Buddhist and Jain tantras? Who are all these gods anyway, and how do they relate to one another? How did a set of antinomian and transgressive practices take root and flourish, not only in palaces and households, but in celibate contemplative communities as well?
These are the kinds of questions that can only be meaningfully addressed by consideration of the social history of tantra, and that is what the book provides. It's a stiff, academic read, and presupposes some familiarity with the subject, but for serious students of the material I can't recommend it highly enough.
A couple minor observations: Samuel analyzes the interplay between India and China at some length, but I would have preferred significantly greater consideration of the possible dialog between India and lands to its west, especially Greece and Mesopotamia. We have compelling evidence for important exchanges there, and if the book truly aspires to help forge a common basis of understanding between the traditions of India and the West, as the book maintains, that's an important place to look.
Additionally, I note that nearly every one of the author's sources are secondary texts written in English. Not being trained in Asian languages, I conject, he had to rely on translators and interpreters. Given the vast body of material he consulted, I don't particularly regard this as a fault, but it's worth noting.