The main message of the book "I Am That - Dialogues with Nisargadatta Maharaj on the Self" is the exploration of the true nature of the self and the ultimate reality. Through a series of conversations between the spiritual teacher Nisargadatta Maharaj and his disciples, the book delves into the fundamental question of "Who am I?" and challenges readers to question their own beliefs and perceptions about themselves and the world around them.

At its core, the book emphasizes the concept of "I am," which refers to the pure consciousness that exists within each individual. Nisargadatta Maharaj teaches that this consciousness is not limited to the physical body or the mind, but is the essence of our true being and the source of all existence. He encourages readers to go beyond their identification with the body and the mind, and to realize the eternal and unchanging nature of the self.

The book also explores the idea of duality and the illusion of separation between the individual self and the universal consciousness. Nisargadatta Maharaj emphasizes that the self is not separate from the world, but is in fact the very essence of it. He encourages readers to let go of their attachments and desires, and to realize their true nature as the pure consciousness that is one with everything.

Throughout the book, Nisargadatta Maharaj challenges readers to look within themselves and to question their beliefs and perceptions. He encourages them to let go of their conditioning and to experience the truth for themselves. The book serves as a guide for readers on their spiritual journey, providing insights and perspectives that can help them in their quest for self-realization.

Ultimately, the main message of the book is that the true nature of the self is beyond words and concepts, and can only be experienced through direct realization. Nisargadatta Maharaj's teachings invite readers to transcend their limited understanding and to awaken to the ultimate reality of their own being.