Following are some of the books and links that can be helpful for a spiritual aspirant.
Adi Shankaracharya, the great illumined teacher of Self and commentator on Upanishads, laid the philosophical foundations
of Advaita Vedanta. In a short life of thirty-two years he wrote many Vedanta treatises and commentaries on all major Upanishads,
Srimad Bhagawad-Gita, and Brahma-Sutras. For a serious student of Vedanta, study of his writings is extremely useful. For more
information on the Acharya and his works, please visit Shankaracharya.org.
To understand the precise gist of Shankara's philosophy and to get his brief life-sketch, please read the book 'Sri Shankaracharya:
Life and Philosophy' by Swami Mukhyananda; the book can be purchased online on
Amazon.com or at Advaita Ashrama. And
Sringeri.net is a project by the Sringeri Matha (Shankara Bhashya) which interlinks the works of Shankara.
Power of Now by Eckhart Tolle has proved to be an influential spiritual guide of modern times for countless people around the
world. It has been translated into many languages and like 'Talks with Sri Ramana Maharshi' and 'I Am That', in this book too, the author
speaks from his own experience, which makes every word in it sound convincing to the reader.
Read more about it at Eckharttolle.com or get
Some more resources:
Advaita.org.uk has essays on Advaita, links to organizations &
free ebooks, ezines, egroups etc.