This book is the best on Jnana Marga or Jnana Yoga. It is a collection of almost everything written or translated by Bhagavan Sri Ramana Maharshi, one of the greatest sages of the twentieth century.
This book can very well be considered the gold standard on Jnana Marga or the Path of Knowledge, just as Narada's Bhakti sutras is for the Path of Divine Love and Patanjali's Yoga sutras is for the Path of Meditation. The book consists of his most famous teachings, 'Who am I?', 'Self-enquiry' and 'Spiritual instruction'. These three chapters are in a question and answer format. This is Jnana Marga simplified and presented to modern age. Maharshi acknowledges that Jnana Marga is not for everyone and so in addition to discussions on Jnana Marga, there are also beautiful summaries of other paths to Self-Realization like Bhakti Yoga (Yoga of Divine Love) and Dhyana Yoga (Yoga of meditation). One can see how the eight steps of Dhyana (Yoga Ashtanga) compares with the eight steps of Jnana (Jnana Ashtanga).
His original writings, Upadesa Undiyar and Ulladu Narpadu follow, which have been translated into very good English. These contain the essence of his teachings.
Next follow translations of important texts on advaita & jnana marga like Atma Saksatkara (instructions of Lord Siva to his son Guha), Devi Kalotarra (instructions of Lord Siva to his wife Parvathi) and major original works of Sankara (including the famous Vivekachudamani). The section on Gita contain the important verses of Bhagavad Gita selected and rearranged by Bhagavan.
Towards the end, Maharshi guarantees Self-realization if one is to read this book and puts its words into practice.
If there is only one book you can afford to read on Sanatana Dharma (the Vedic Religion or Hinduism), this is the one.