In no order other than alphabetical...
The original edition dates to his pre-Adidam Revelation days, and can perhaps be considered a purer source of inspiration than the more recent Intermediate-Zone inflationary material, although the latter packs a stronger astral "charge" The Dawn Horse Testament of Heart-Master Da Free John. The Dawn Horse Testament of the Ruchira Avatar: The Testament of Divine Secrets of the Divine World-Teacher, Ruchira Avatar Adi Da Samraj is the 2005 edition linked to on Amazon; I only have the 1985 edition, so I don't know how much this newer edition has been modified.;
A classic of nondual wisdom by Nisargadatta Maharaj, an authentically enlightened adept. The material may be technical and heavy-going to someone with no familarity with Vedantic jargon, but otherwise it is very straightforward. Although Nisargadatta does not have the avataric presence of Ramana Maharshi, he is presenting the same universal truth.
Nitya Sutras: The Revelations of Nityananda from the Chidakash Gita by M. U. Hatengdi, introduction by Swami Chetanananda
This wonderful book appears to be out of print. It provides a collection of aphoristic sayings of Nityananda, which truely convey the Enlightened state of consciousness. These can be used as triggers for jnana (Insight). Nityananda, was a rare totally enlightened master, similar to, and a contemporary of, Ramana Maharshi. If you can get it second hand, it is wortth owning and reading. Some of this material has been posted online.
The Power of Now is a good example of a Western "New Age" book that presents the same basic message as the non-duality teachers of the East, but in a very simple and non-jargonesque manner. I find this material too lightweight for my tastes, but there are many who are inspired by it. Oprah Winfrey copped a lot of flack by promoting Tolle (hence rocketting his books to the best seller lists), as redneck conservative religions automatically classify him as a "cult", while not applying the same discrimination or criticism to their own fundamentalist religion. Tolle's material is far superior to The Secret and the Da Vinci Code, although all can be considered Popular New Age (the the Da Vinci Code as more mainstream than the others, but with the same theme of "seeking for truths"). I cannot say that Tolle is Realised or at the level of someone like Nisargadatta, Nityananda, or Ramana, but certainly he has tapped into some basic insights. For a more sophisticatred Western approach, see A. H. Almaas.
This book amde a big impression on me when i read it, although i never got past the first half or so. A condensed summary of the Pratyabhijna/Trika dynamic nondual teachings of Kashmir Shaivism, which are very different to the static nonduality of pure Advaita Vedanta. Conventional Advaita (and Advaita-like teachers like Ramana) emphasise the Self as the true Reality and foundation of all being within. This indeed is the basic insight that conveys Enlightenment (through jnana). But Pratyabhijna adds to this the dynamic element of Shakti, and describes how this is accessible even in the microcosm of individual consciousness. I would consider this state to be intermediate between the basic nonduality and the more profound Integral Yoga synthesis of Sri Aurobindo, which goes beyond (while still including) nonduality entirely.
Sankara tended to write large commentaries on the various Upanishads and other Vedic/Vedantic sacred books. However his Vivekacudamani represents a marvellous concise summing up of the spiritual philosophy of Advaita Vedanta; the central Indian tradition of nonduality that Sankara founded in the 8th/9th century. More than a quarter of a century ago, I bought a small little paperback edition in India, which gives the essential text only. Only a few editions and translations are linked to here, and it is not claimed taht these are better or worse than other translations.
The edition from the Aurea Vidya Foundation, translated with commentary by Raphael, gives the text but also padded out with commentary. Raphael is described as an author abnd master of both Western esoteric (hence his name) tradition as well as Vedanta. His commentary provides a highly lucid and very easy to follow explanation of the original text. While such commentary isn't essential if you are already familiar with Advaita philosophy, this or a similar explanation would be helpful if you aren't.
I haven't seen a preveiw of the translation by Swami Turiyananda, but I would assume this to be a traditional Indian one, since it is from the Sri Ramakrishna Math.
There is also a free online edition with headers added but no commentary: Vivekachudamani by Adi Sankara, translated by John Richards. No doubt there are many more translations around. More books on or by Shankara at this page.
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