These essays are intended for readers who are familiar with at least the basics of Rudolf Steiner's Anthroposophy. Much of Steiner's writings and lectures can be found online (in English) at http://www.elib.com
(The original language is German. The German texts can be found online at many places. The translations are unavoidably imperfect.) Those new to Anthroposophy would probably do better to start reading with the "basic books" rather than the lecture transcripts. The quality of these transcripts varies greatly, and Steiner's lectures were mostly intended for listeners already familiar with his basic concepts. (There were some exceptions.) Generally, he was carrying on "conversations" of sorts with his audiences (again, there were some exceptions), and the lecture transcripts should be viewed in that context. Most lectures were never intended for the general public, and Steiner only reluctantly, toward the end of his life, permitted the publication of the transcripts -- but with this "caveat":
"Only it will be necessary to remember there are errors in the lectures [transcripts] which I did not revise."
Therefore, the new reader should most probably start with the "basic books" written by Steiner and intended for the general public. And those reading in English should remember that the crucial terms are translated from the German, and that the English words are sometimes not used in the ordinary sense. The German terms are used in senses closer to ordinary German usage, but, even so, they are sometimes used in special senses and constructions that are difficult even for native German speakers.
There are varying opinions as to which Anthroposophical books are most basic. IMO probably the most basic is Theosophy, which explains a number of Steiner’s key terms. Also basic is Occult Science, an Outline, which goes into Steiner's basic cosmogony, and also his Path of Cognition. The book Knowledge of the Higher Worlds and its Attainment is concerned almost exclusively with the Path of Cognition.
These books are not easy reading. They need to be read slowly and carefully. Some sentences will need to be read several times over. It is helpful, wherever possible, to form clear mental pictures of whatever Steiner is describing.
Even more difficult is Steiner's (probably) most basic philosophical-epistemological book, The Philosophy of Freedom (also sometimes called The Philosophy of Spiritual Activity). The original edition was written in the context of Nineteenth Century Teutonic academic philosophy, and it was not understood at first even by that readership. The edition we have now (from 1918) contains only brief references to Steiner's "occult" teachings, and is more of a training manual than a "book" in the usual sense. Even many Anthroposophists still have difficulty with this book, but it is basic to an understanding of Anthroposophy. Readers can get some help from Otto Palmer's compilation: Rudolf Steiner on his Book The Philosophy of Freedom [Anthroposophic Press; 1979] and from Georg Kühlewind's Stages of Consciousness, Chapter One [Lindisfarne Press; 1984]. -- But really, it is almost futile to read this book for information; the point of the book is to change one's way of thinking, one's mode of consciousness.
Some people consider Steiner's book Christianity as Mystical Fact (also called Christianity and the Occult Mysteries of Antiquity) to be basic, but IMO it is not as fundamental as the other listed here. But it could serve as a preface to Steiner's elaboration of Christology.
(Perhaps at this point it would be useful to explain about the word "occult". Some people may be put off by that word, but sometimes unnecessarily so. The word is Latin, merely meaning "hidden"; it does not mean "Satanic" or "black-magickal". The German word that Rudolf Steiner used is "geheim", meaning "secret". Some societies keep their knowledge and practices secret for good purposes, others for evil. There have been pretty much always societies that kept some knowledge secret, both in pre-Christian times and after. For instance, in ancient Egypt the secret societies were well-known and publicly active, and outwardly visible, as is shown by the Pyramids. In ancient Greek times the secret knowledge was called the "Mysteries". In Christian times some religious knowledge has remained secret, following the saying of Jesus: "Do not give what is holy to the dogs; nor cast your pearls before swine, lest they trample them under their feet, and turn and tear you in pieces." Some things are too holy to be given to the public, and common sense tells us not to give gasoline and matches to children. Of course, some societies keep their knowledge secret for nefarious purposes; even today these kind of secret societies are especially active and influential in politics. But still, some knowledge is kept secret for good purposes. So, the word "occult" is morally neutral; it is not necessarily equivalent to "Satanic"; it may be equivalent to "esoteric", but does not in itself imply either good or evil.)
I hope that the essays contained in this site might be useful for readers who are adequately prepared.
All of the essays can be found in the menu at the top of each page.