'The Secret Doctrine' is a classic of Theosophical mysticism and esoteric doctrine. This book is a must-have for anyone seriously interested in philosophy or religion.
The name "Theosophy" is derived from the ancient Greek for "Divine Wisdom," and is used by the Theosophical Society (founded 1875) to describe a synthesis of philosophy, ancient spiritual mysticism and modern science.
But be aware that Theosophical works such as 'The Secret Doctrine' present a rather controversial view of the universe, and the reader should be prepared for a long process of in-depth study and consideration of ideas that do not follow the normal mainstream of traditional dogma.
The unorthodox philosophy contained in 'The Secret Doctrine' has been abused by such infamous figures as Adolf Hitler and others. But great personalities such as Alexander Graham Bell, Henry Ford, Thomas Edison, Albert Einstein, and Mahatma (Mohandas) Ghandi have also been associated with Theosophical studies. This work is a classic among intellectuals of the 19th and 20th centuries.
'The Secret Doctrine' describes the Universe in terms of Pantheism (which refers to the intimate connection between Divinity and Nature, not "multiple gods" as is commonly misunderstood), and in this way, explains in logical terms a common set of fundamental principles behind many of the religions of the world. Theosophy shows the relationship of the Inner Essence of the soul of mankind to the Divine Source, and makes clear the "how" and "why" behind the Golden Rule and all the moral teachings of the various religions.
In light of the Theosophical explanations, traditional morals, ethics, and spirituality become more than a set of arbitrary dogmatic rules. For those who look closely, even such fundamental Christian principles as salvation and sacrifice become clear.
In addition to 'The Secret Doctrine,' other works of Helena Blavatsky, as well as William Q Judge, Annie Bessant, and Charles Ledbeater provide insight into this fascinating area of philosophy.
I would particularly recommend 'Man, Visible and Invisible,' 'Isis Unveiled,' 'Thought Forms,' 'The Hidden Side of Things,' and 'Golden Precepts.'