I bought them from Taka Kinokuniya under Buddhism section.
About the Golden Letter. It is a very important book. It contains
the transmission by Garab Dorje to his principal disciple before
dissapearing in the body of light. This book also attempt to
connect western concept of ascension with Dzogchen teaching. Here
we can see that how far modern Church-based Christianity has gone
off course. No offence to anyone. But Western mysticism is still
very similar to Buddhism especially Vajrayana.
About Dzogchen, to me, it is a very mysterious teaching. There are
2 versions of Dzogchen... Buddhist and Bon. Both are essentially
the same. However, Bon teaching originates from a Buddha before our
historical Buddha. Accordingly, this Buddha call Tonpa Shenrab
lived thousands of years ago before our historical Shakyamuni
Buddha. Also, Dzogchen teaching is said to be taught in 13 other
star-systems as well, to both humans and non-humans.
It seems periodically, there are lineages of physical Buddhas that
teaches the same thing. Regardless, all teachings are said to
originate from one source Dharmakaya Samanthahadra.