The following books are the most useful ones I know for somebody who wants to know what to do to realize the Self. The first book on the list — I Am That — is the most useful but it probably won't be intelligible until you've worked at self-enquiry for a while.

I Am That by Sri Nisargadatta Maharaj, edited by Maurice Frydman.

The Path of Sri Ramana Part One by Sri Sadhu Om. This book can be downloaded for free from its translator's website.

The Most Rapid and Direct Means to Eternal Bliss by Michael Langford.

Beyond the Mind by Dada (Dattaram Gavand)

Experience and Philosophy by Franklin Merrell-Wolff

The Cloud of Unknowing and the Book of Privy Counseling (author unknown)

Talks With Sri Ramana Maharshi edited by Sri Munagala Venkataramiah

Annamalai Swami: Final Talks edited by David Godman.

Living by the Words of Bhagavan edited by David Godman.