What books should be read?
The following books are very helpful -
1. Scriptures - Bhagavad-Gita and Upanishads:
Srimad- Bhagavad-Gita Translated by Swami Swarupananda.
Kathopanishad, Mundaka Upanishad, Ishopanishad by Swami Gambhirananda.
Very Special Reading: The Gospel of Sri Ramakrishna.
2. Works of Swami Vivekananda:
1. Karma Yoga, Raja Yoga, Jnana Yoga and Bhakti Yoga or if possible read the Complete Works of Swami Vivekananda.
3. Divine lives:
Reminiscences of Swami Vivekananda by the Eastern and Western Disciples.
They Lived with God, God Lived with them, How to Live with God all by Swami Chetanananda.
Sri Ramakrishna and His Divine Play Translated by Swami Chetanananda, Sri Sarada Devi and Her Divine Play by Swami Chetanananda or Holy Mother Sri Sarada Devi by Swami Gambhirananda.
For further reading browse - http://www.vivekananda.net/
It is better to read printed book to avoid the possibility of getting lost in the wilderness of the Internet.