The book 'History of Indian Esoteric Buddhism' is recommended for Esoteric practitioners who want to deepen their learning

As this book was compiled as a summary of a lifetime of work by a great scholar who spent many years traveling the world in pursuit of the history of Esoteric Buddhism, I read it with great respect.
It is a charming anecdote that his own books and papers were translated into English and Chinese, and that he was actually a famous figure among Buddhist scholars in various countries, though he himself was completely unaware of this and was surprised to be welcomed at international academic conferences and other events.

This book is packed with the research results that such a great Buddhist scholar has accumulated over his lifetime.

It is recommended for those who are interested in or practicing Esoteric Buddhism (in Japan, Tibet, or Bhutan) and who want to delve into the historical background of Esoteric Buddhism.

The content of the book is apparently intended for lectures to university students, but as it is difficult even for me, a veteran (or so I consider myself) who has been practicing Esoteric Buddhism for many years, I think it might be too advanced and challenging for students.

When reading it from the perspective of someone who knows and practices Esoteric rituals,

I often find myself realizing,

So that's where that comes from!

For example, the Buddha family, Lotus family, and Vajra family that appear in the Goshin-ho (Self-Protection Ritual).
It seems that by the time Esoteric Buddhism began, they were already being worshipped as this set of three.

Whether the ritual came first, or the worship came first and the ritual was born from it, I do not know, but the way the faith was practiced in India at that time still remains in the Northern transmission of Esoteric Buddhism of the Shingon sect and Agon-shu (the basis of whose rituals is the Sanboin-ryu Kenjin-kata), and is still practiced today.

I feel a depth of emotion that I cannot express when I realize that I am in the midst of such a flow of time and historical romance.

Incidentally, the same elements remain in Tibetan Esoteric rituals, although they are not grouped as the Goshin-ho. The mantras are understandable because, despite the differences in Japanese and Tibetan pronunciation, they are generally the same.
↓ The reference book is probably this one. Please note that it also contains some very worldly content.

It also provides detailed explanations of later Esoteric mandalas and theories that were not transmitted to Japan.

The transformation of Esoteric thought and the expansion of mandalas are extensions of the Vajradhatu (Diamond Realm) teachings, but they take a mysterious turn that makes you wonder, 'How on earth did you guys end up like that?'

As I read through the history of mandala expansion and the thought of later Esoteric Buddhism in this book, I can only conclude that the brilliant but narrow-minded bureaucratic elite monks of the time fabricated sophistry to build their track records, leading to expansion upon expansion.

That is my impression, but I think this book will be one where what you learn and how you feel will differ depending on your own relationship with Buddhism.

There is no doubt that it will deepen your academic understanding of Esoteric Buddhism.