Ommani you seem pretty well versed and mention Buddhism frequently, what do you think of this guy's book (it's short) ?
[Internet Archive Link = Dark Zen by Zenmar (1995)]
It's hard for me to say without reading the whole book, but I'm not sure about the extent to which it corresponds to Buddhism/Zen, as it is commonly understood. In essence, when I see someone dismissing an entire tradition and saying that their interpretation of it is the correct one, my inclination is to look elsewhere, especially when the person in question doesn't appear to represent an established lineage.
With regard to Zen, the books,
Zen Mind, Beginners Mind and
Not Always So, by Shunryu Suzuki are well known classics of the genre that I can recommend.
When it comes to Vajrayana (Buddhist tantra)/Tibetan Buddhism, Anam Thubten is probably my favorite teacher. He's got lots of great books, including one that is specifically about Dzogchen, which is generally considered the most advanced (non-dual) form of Tibetan Buddhism, corresponding to Mahamudra (Dzogchen is part of the Nyingma school of Tibetan Buddhism, while Mahamudra is related to the Kagyu school).
Chogyal Namkhai Norbu is another Tibetan Buddhist teacher you could look into, who focused largely on the Dzogchen tradition.
Pema Chodron is another great teacher who takes the vast teachings of Tibetan Buddhism, and brings them down to earth.
I've actually been listening to the audiobook of The Magic of Vajrayana that
@northape recommended, which I really like so far. I've also been listening to his book, Reflections on Silver River: Tokme Zongpo's Thirty-Seven Practices of a Bodhisattva, which pertains to the Mahayana teachings of Buddhism.
Two cross-references for the aforementioned book would be The Heroic Heart: Awakening Unbound Compassion - A Modern Commentary on the Thirty-Seven Verses on the Practice of a Bodhisattva, by Tenzin Palmo
and
The Heart of Compassion: the Thirty-Seven Verses on the Practice of a Bodhisattva by Dilgo Khyentse
I haven't read, A Trackless Path, by Ken McLeod, which I saw northape had recommended in your post, but have the audiobook and plan to give it a listen - this book deals with the topic of Dzogchen and is a commentary on a poem written by renowned, 18th century Dzogchen master, Jigme Lingpa.
For a little context, Tibetan Buddhism is organized into the foundational teachings of Hinayana (analogous to Southeast Asian, Theravada Buddhism), Mahayana (focusing on teachings of emptiness and compassion), Vajrayana (Buddhist tantra), and Dzogchen/Mahumudra (ultimate/non-dual teachings/practices, through a Buddhist lens). I suppose that Dzogchen/Mahamudra could be included under the umbrella of Vajrayana, yet are also distinct from the kinds of tantric (deity yoga) practices that I associate with Vajrayana Buddhism.