Shankara ruthlessly deconstructs the "I-am-the-body" idea. He uses the analogy of the rope and the snake: In darkness, you see a snake (the world); with light (knowledge), you see it was a rope (Brahman) all along.
The Vivekachudamani (Crest-Jewel of Discrimination) is a classic 8th-century Sanskrit poem attributed to Adi Shankaracharya. It is arguably the single best introductory text for non-dual philosophy. vivekachudamani pdf english