That in whom all beings reside and who resides in all beings, who is the giver of grace to all, The Supreme Soul of the universe, the limitless being — I am that. Amritbindu Upanishad.
This is perhaps one of the most relevant and revealing books of Advaita Hinduism, made up of a collection of 101 conversations with visitors to the jñani Sr. Nisargadatta Maharaj (April 17, 1897 - September 8, 1981).

(Image: Nisargadatta Maharaj)
He offers, before their questions, his vision of the reality of the Self which later gave rise to Nisarga Yoga.
As he reveals, the reality came to him once his master (Siddharameshwar Maharaj in 1933) told him: “You are nothing but your self. Attend to the <>.” Trusting this, he acted accordingly and stopped worrying about what was not his self, nor his (1). He recommends discarding everything that “we are not” as a way to reach what we truly are.
<> is an enjoyable and refreshing read to digest little by little (it’s almost 500 pages), where the master removes all the prisons with which we identify.
“You are not your body, you are not your mind, nor are you what it thinks” (but you are everything, the self, the Being).
Yoga is the tool to undo the confusion between the limited and the unlimited, which are mistaken for the same, and to help us discover what we really are (2).
Interlocutor: Doesn’t death undo this confusion?
Maharaj: In death only the body dies. Life does not die, consciousness does not die, reality does not die. And life is never as alive as after death. (3)
He also offers the technique of asking oneself “Who am I?” (4), promulgated in his case by Sri Ramana Maharshi.
Mah: What is wrong with striving? Why seek results? The effort itself is your real nature.
Mah: You make it painful by seeking results. Strive without seeking, strive without greed.
Mah: Which God are you talking about? What is God? Isn’t it the very light by which you ask the question? The “I am” itself is God.
Ultimately, it is a must-read book, highly recommended, you can find a Spanish translation here (pdf).
FOOTNOTES
1 See dialogue 51 of the book for the full description.
2 What in the West we would identify as the spirit.
3 In full agreement with the Bhagavadgita, see conversation 5 of the book
4 See details in conversation 21 of the book