Shankara Bhagavatpada (~788 - 820 CE) was a true master of the highest caliber who expounded the ancient teachings of the Bhagavad Gita, the Principal Upanishads, and the Brahma Sutras, usually known as Prasthanatraya in an ingenious way.
This anthology of nearly 1900 selections arranged in 160 topics from different English translations of the Prasthanatraya and other sources should assist any serious seeker regarding modern Vedantic exegesis.