Shankara formulates the doctrine of Advaita Vedanta by validating his arguments on the basis of quotations from the Bhagavad Gita PDF and other Hindu scriptures. A large portion of his works is polemical in nature. The commentaries serve to provide a consistent interpretation of the scriptural texts from the perspective of Advaita Vedanta.
The philosophical treatises provide various methodologies to the student to understand the doctrine. A lot of editions of the works of Adi Shankara are available. Sri Sankara Granthavali – Complete Works of Sri Sankaracarya in the original Sanskrit, v. Originally published from Sri Vani Vilas Press, Srirangam, 1910ff.
1-4, Basumati Sahitya Mandira, Calcutta, 1995. Sankara’s bhashyas on the Katha, Mandukya, Taittiriya, Chandogya and Brihadaranyaka Upanishad, with Anandagiri’s Tīkas and other sub-commentaries. A collection of twelve prakarana granthas, with commentaries. A Bouquet of Nondual Texts, by Adi Sankara, Translated by Dr. Ramamoorthy and Nome, Society of Abidance in Truth, 2006. This volume contains the Sanskrit original, transliteration, word-for-word meaning and alternative meanings, and complete English verses.