A short commentary on Adharva veda Upanishads- Amrita bindu Upanishad and Kaivallyopanishad.
One hundred and thirty or more Upanishads. Of these one hundred and odd Upanishads, only ten have been fully commented upon by Sri Sankaracharya, whose commentaries on Upanishads are the earliest extant. It is only these ten Upanishads and four others that have been cited as authorities by Sri Sankaracharya in his commentaries on Brahma-S'utras, and among them alone are those few that can be traced to the current Vedic schools. This fact as well as a striking difference in diction and subject-matter between these Upanishads and the rest has led some critics to regard the former alone as genuine Upanishads and the latter as mere imitations if not worse. Without, however, venturing the bold opinion that this view is altogether unfounded, one may still hold that even those Upanishads which Sri Sankaracharya has not commented upon or otherwise noticed may justly be allowed the title, as they conform to the accepted definition of the term. The great commentator derives the term from three words upa (near), ni (quite) and sad (to go, to perish, to waste away) and explains that the word means Brahma-vidya, the Spiritual Wisdom which, by leading its Devotee very near to Brahman, brings about the final extinction of misery by eradicating it and burning up its very seed, avidya. And in This widest acceptation of the term, the title has been extended by later writers to such Works as the Bhagavadgita, which treat of Brahman and the means of attaining Divine Bliss.
This elasticity in the application of the term does not altogether militate against the Brahmanical doctrine of revelation; for, while Holding that Vedas including Upanishads are eternal as embodying the eternal truths which, though not accessible to the mind of the ordinary man, are yet within the ken of the spiritual vision of the divine sages who can read them as it were recorded in the pages of super physical nature, the orthodox Brahmanism .admits the possibility of sages and even the Divine Being revealing at different ages for the -guidance of people so much of truth concerning transcendental matters as may be necessary -for their spiritual progress, in the language of the people to whom the teaching is addressed. Unless, therefore, the application of the term is restricted to works of a particular age in the historical period, the title cannot be refused to the Upanishads in question.