This is a fantastic and monumental book. Too bad it seems to be out of print or otherwise unavailable for some reason. I supposed the academic tone and 700+ pages results in few readers. If you are seriously interested in Tibetan Buddhism, however, and not just in accumulating as many blessings as possible but in really understanding where it came from and how it developed, this is an excellent book. After reading something this huge and detailed, you might think you'd then have a fair grasp of what's going on, but the effect is exactly the opposite: you realize just how ignorant you are of the vast panoply of practices, roles, techniques, and so forth Tibetan Buddhism offers, or has offered, over the last 1500 or so years.
The first part is 'anthropological' and kind of drags, unless you're really into who ruled who and who had more yak-power in various areas of Tibet pre-Buddhism. It picks up in the middle portion, where Samuel gives a massive overview of the many 'roles' (monk, yogin, lama, etc.) one might play in Tibetan Buddhism and traces his major clerical (monastic) vs. shamanic (yogic) thesis re. Tibetan Buddhism.
The last part is I think the best, tracing the history of Tantra from India into Tibet, and here insight follows insight on almost every page. There may be other books which trace the co-mingling of tantra, shamanism, Tibet and India, but I don't know of them; in either case, this is the most insightful book about tantra and its connections with shamanism that I have yet read.
One other thing is that Samuel considers several times the origins of the tantras themselves. Tibetans of course insist these were the true words of the Buddha, held 'Elsewhere' until humans were prepared for them, and then transmitted via various deities and/or ecstatic visions to adepts and written down for our benefit. Most scholars wave their hands and say, clearly these are later works, there is no possible way they really have any connection to the historical Buddha. Samuel's take on the whole thing - basically, that tantric visions are a variation on the tried and true practice of shamanic revelation - provides the only plausible, not to mention interesting, resolution of I have ever come across to this debate. He also points out how the Buddha's own revelations are highly shamanic in nature, which I found interesting.
In short, this enormous and scholarly book (the bibliography alone is a very interesting 60+ pages) has much to offer in terms of just pure information and insightful commentary, at least in the final two-thirds. Very highly recommended to anyone with a serious interest in Tibetan Buddhism.