Bhagavad Gita.
I found it on Gutenberg, where you can read it online for free. Thanks for the recommendation!
Right now, another Gutenberg book I am reading is the Tao Te Ching (the central text of Taoism, also called Daoism) – really a compilation of texts written between late 4th and 3rd centuries BCE. Traditionally, it is attributed to the philosopher Lao-Tse.