The four books are the four major collections of Said Nursi’s treatises: The Words, The Letters, The Flashes and The Rays.
Among these, The Words explains and proves the fundamental matters of belief, such as God’s existence and unity, the manifestation of the divine names and attributes in creation, the resurrection of the dead and the life of the hereafter, prophethood, the miraculousness of the Qur’an, the angels, and the immortality of man’s spirit, together with such questions as the true nature of man and the universe, and man’s need to worship God. The Letters is largely a compilation of answered Said Nursi gave to his students. These letters cover issues from daily life of Said Nursi to explanations of the truths of belief and mysteries of the Qur’an, from nationalism to sufism and various treatises on how to read the Book of the Universe. The Nineteenth Letter, which describes the miracles of Prophet Muhammed (Peace be upon Him), is unquestionably the core of this book.
The Flashes (a.k.a. The Gleams) continues to expound the fundamental truths of belief, existence and science, the Practices (Sunna) of the Prophet Muhammad (Peace be upon Him), and various topics relating to the social and personal lives of Muslims, including the treaties on brotherhood and sincerity.
The Rays is the final collection of the Risale-i Nur containing both argumentations on the existence of God and Nursi’s defences in the courts is a guidance for the Risale-i Nur’s way of life and for a spiritually enlightened life at the same time. The dual cores of the collection are The Supreme Sign (The Seventh Ray) which provides a wholistic testimony to God’s necessary existence and unity, and The Shining Proof (The Fifteenth Ray) which not only summarizes the argumentations on God’s existence put forward in the Risale-i Nur, but also adds short treatises on spiritual realms.