Upanishads, Bhagavad Gita and Brahma Sutra are together known as the root texts of Vedanta. If you want one book, it just has become the book. So I said to that kid, that if you study the Gita, you study the Vedas and Vedanta,. All the essential teachings you know. In fact, in one of the verses praising the G Rita, it is said the Upanishads are the milk cows, the cows.