The idea that Śaṅkara was a Brahmin from the south who taught and wrote mainly in the north, who gathered many pupils about him, who won fame travelling about and engaging in debates and who was a devotee of Viṣṇu can be supported from the surviving writings of Śaṅkara himself and his early followers.
The picture drawn in the Śaṅkara Digvijaya of Śaṅkara travelling far and wide and gaining fame as a Teacher and debater can also be supported from the same sources.
That Śaṅkara was an incarnation of the deity Śiva receives no support from contemporary sources. On the contrary, a certain predilection for Viṣṇu has been detected in Śaṅkara’s own writings and in those of his immediate pupils and followers which militates against the possibility of any contemporary belief that he was an incarnation of Śiva. For instance, Śaṅkara himself identifies Hari and Nārāyaṇa (names of Viṣṇu) with the Absolute in his Brahma Sūtra commentary, but does not mention Śiva in this way.
But if his early followers did not regard him as an incarnation of the deity, they certainly regarded him as a Teacher of quite exceptional importance and magnitude.
Certain passages in his commentaries suggest that he had the capacity to write beautiful devotional poetry if he had wished, but in the verse part of the Upadeśa Sāhasrī, the only surviving verse work of certain authenticity, the beauty derives from the content rather than from the form throughout.
The groundwork for securing criteria for distinguishing between the authentic and inauthentic works has been done by Professor Hacker. The authenticity of the Commentaries (Bhāṣya) on the Brahma Sūtras and on the Bṛhadāraṇyaka, Chāndogya, Taittirīya, Īśa, Aitareya, Kaṭha, Praśna and Muṇḍaka Upanishads is not questioned by the vast majority of authorities.
Professor Hacker’s methods, has removed all reasonable doubt as to the authenticity of the commentaries on the Bhagavad Gītā and on the Māṇḍūkya Upanishad with Gauḍapāda’s Kārikās, as also of the two commentaries on the Kena Upanishad. It appears also that there is no reason to doubt the authenticity of the commentary on the Adhyātma Paṭala of the Āpastamba Dharma Sūtra,
Excluded (and it is very important to exclude them if one wants clarity about what Śaṅkara actually said) are such popular favourites as Viveka Cūḍāmaṇi, Ātma Bodha, Svātmanirūpaṇa, Aparokṣānubhūti and Śata Ślokī, which belong to an altogether later age. It is also unsafe to use any of the devotional hymns attributed to Śaṅkara’s name as guides to his doctrine. For instance, the two of them with the best prima facie claims to authenticity are the Dakṣiṇā Mūrti Stotra with a commentary ascribed to Sureśvara and the Hymn to Hari with a commentary ascribed to Ānandagiri. Both works, however, have dubious features.
The present anthology is accordingly based on the Commentaries to the Brahma Sūtras, the Gītā, the Kārikās of Gauḍapāda and to the Adhyātma Paṭala of the Āpastamba Dharma Sūtra, and on the individual commentaries to the classical Upanishads.
~A. J. Alston, Absolute, pp55-62
Dennis Waite (from Back to the Truth):
A. J. Alston (died 2004) was the brilliant translator of “The Method of the Vedanta”* (see [below]). His ability to render the often abstruse philosophical arguments of Shankara into comprehensible and readable English is without parallel in my experience. Accordingly, this set of books – “A Shankara Source Book Vols. 1 – 6” - is invaluable to serious students of Advaita. I have only read one of these - Vol. 2 Shankara on the Creation (Ref. 335) - but am prepared unreservedly to recommend them all on the basis of this. Each book is divided into clear sections and sub-sections. Each topic is introduced and explained by the author, who then selects relevant passages from Shankara’s text which address the topics. It took Alston 37 years to complete this task and Advaitins everywhere can now reap the rewards.
* The Method of the Vedanta: A Critical Account of the Advaita Tradition by Swami Satchidanandendra, translated by A. J. Alston (Ref. 24). This is a huge book, requiring considerable commitment but, if you want to understand clearly what Shankara believed and how his message has been modified or even distorted by subsequent interpreters, then it is indispensable reading. Shankara’s essential method is presented as that of adhyAropa - apavAda, attribution and subsequent denial. His commentaries on the prasthAna traya are examined in detail. Then, following a brief look at pre-Shankara Advaita, there are chapters on each of the major teachers and schools that followed him, in which the same topics are re-examined and the differences outlined. Fortunately, the translation is by A. J. Alston - see below - so is always understandable.