Mandukya Karika

The _Mandukya Upanishad_ is the most concise of the Indian philosophy texts. It's considered an exhaustive overview of _Advaita Vedanta_, which is basis for non-dualism and has inspired many Western philosophies.

It usually comes with the _Karika_, which is a short commentary on it written by Swami Gaudapada, one of the famous original thinkers behind Advaita.

It's filled with great analogies for understanding non-dualism, along with several overly religious or complex descriptions that could benefit from simplification.

You can find it here - be warned, it's a challenging read.