The Vedanta system is built primarily on the Prasthanatraya the three foundational works that take one to the final goal of life (prasthana). They are the Upanishads the Bhagavad Gita and the Brahma Sutras.
Of these three the Upanishads being the Sruti the cardinal Scripture of Hinduism are of utmost importance.
The Brahma sutras being a systematisation of the philosophy of the Upanishads helps in building up the metaphysical frame of Vedanta.
The Bhagavad Gita is an extremely popular and highly venerated work. The philosophy of the Bhagavad Gita is similar to that of the Upanishads so much so that it has been described as themilk of thecows of the Upanishads.
The present text contains brief synopsis of the above mentioned three foundational works: the philosophy the authors the books commentaries about the work/book etc.