Essential Knowledge is a summary compilation project of the entire series of published books by Carlos Castenada from teachings delivered to him during his apprenticeship to Mexican sorceror don Juan Matus - compiled, ordered and edited for clarity and flow. (There may also be further additions in future from his other apprentices as relevent.)
The intention here is to provide the essence of information regarding the main topics of interest to shamanism in a more compressed and easily digestable form, as for a number of reasons the original series of twelve books contains a great deal of both repetition and scattered one-off tidbits.
The advantage of this approach is that the reader will gain an accurate overview of the ideas presented, the disadvantage is that there are many stories and experiences that cannot be included for reasons of length. However, it does provide an unsurpassed introduction to shamanism, and should the reader be interested in further study enable them to place further more detailed reading in a greater context.
In attempting to preserve the original style of writing as best as possible, I have made very few additions or alterations to the sentence structure of the text. It has mostly been a reordering effort, albeit a voluminous one. But these things matter little anyway, I believe the quality and the depth of the information speaks for itself.
Of course there are all sorts of critics out there so I will say just this: Shamanism is an intensely practical affair, and any description thereof no matter how well formulated is all but a vague map. Until you set out on your own voyage of discovery into the unknown, there is no point casting any judgement over the accuracy of the original information and its sources.
So why not find out for yourself first? I hope you enjoy the adventure in awareness that lies before you!
PS. The last book in the series, Magical Passes, cannot fully be included in this summary as it is primarily an pictorial instructional manual in a series of physical and energetic exercises. These are also now known as Tensegrity, a series of movements used by shamans for developing intent, with positive effects on your physical and energetic well-being. These are probably easier to learn from the series of Tensegrity DVDs and the Magical Passes Video created for this purpose rather than from the book form.