To create such a list you must first define what the goal of a spiritual book is intended to accomplish.  Perhaps Meher Baba said it best when he stated, “The real meaning of religion is to know God, to see God and be one with God.  Everything else about religion is an exercise in rites and rituals.”  To me there are 3 important considerations to be mentioned amongst the best classic spiritual books: 

1. For most truth seekers, the goal for any path of spiritual study is to achieve a state of enlightenment in which the seeker and that which is sought are united as One.  Some books either autobiographical, non-fiction, poetic and even fictional seem to guide the reader and point them in the direction more clearly than others.  After all, most would agree that all spiritual books are merely signposts and cannot deliver one to the Goal itself.
2. Classic spiritual books also are not limited to any one religion rather they are able to cross religious lines and transcend them all together.  In order to be considered as one of the best spiritual books of all time, it must present a message that can be applied in any generation or time.  Classifications such as new age, eastern teachings, esoteric, metaphysical, mystic, Christian theology, Middle Eastern, personal transformation, enlightenment, body mind and spirit or any other such label may be convenient identifiers but cannot capture the full essence of a truly spiritually uplifting body of work.  Religious teachings may be confined but spirituality is universal.
3. Lastly, it is my opinion that you cannot teach what you do not know.  There are those who try to venture into spiritual subject areas that they truly know nothing of from personal experience.  They may be good researchers, regurgitators or parrots but they do not live or embody that truth which they speak of.  Not every spiritual author must be a guru or have attained the level of a Buddha, however, they should have attained some level of realization that comes from within and not just hearsay. 

So with these variables in mind I shall share with you spiritual treasures that have not only impacted my personal life, writings and spiritual growth but millions others like myself.  Some of the books achieve the intended goals in various methods.  Some are easier reads than others.  Some are very detailed and lengthy while others may be simpler and yet stir the same inspiration.  In no particular order here are my top list spiritual books: 

• "Autobiography of a Yogi" by Paramahansa Yogananda
• "God talks with Arjuna: The Bhagavad Gita Royal Science of God-Realization" by Paramahansa Yogananda
• "The Second Coming of Christ: The Resurrection of the Christ Within You" by Paramahansa Yogananda
• "The Holy Science" by Sri Yukteswar
• "The Power of Now" by Eckhart Tolle
• "A Course in Miracles" by Marianne Williamson
• "Conversations with God" by Neale Donald Walsch
• "A Systematic Course in the Ancient Tantric Techniques of Yoga and Kriya" by Swami Satyananda Saraswati
• "Journey of the Soul: Day One" by Lateef Terrell Warnick
• "Raja Yoga" by Swami Vivekananda
• "The Aquarian Gospel of Jesus the Christ" by Levi Dowling
• "The Kingdom of God is Within You" by Leo Tolstoy
• "The Prophet" by Kahlil Gibran
• "The Holy Vedas" Vyasa
• "The Book of Secrets of Enoch" by Enoch
• "Beyond Belief: The Secret Book of Thomas" by Elaine Pagels
• "Buddha: A Story of Enlightenment" by Deepak Chopra

In truth, there are a lot of great spiritual books out there but as one who has spent decades reading and studying various teachings you come to realize one day that you can go on researching and reading forever.  The goal to self-realization isn’t in intellectual knowledge but through awareness and higher consciousness.  Ultimately, all these spiritual books share a message based upon the mystic premise “the kingdom of God is within you!”  These magnificent spiritual readings are only meant to provide you with techniques and instructions on how to look within to achieve “union.”  What you experience will be based upon your aspiration, determination and practice.  In the meantime, how you treat others, your love and unselfishness will help to expand your consciousness with the world around you but your heart’s yearning can only be answered by the Source within.  The rest is up to you…