I had read the Yage letters many years ago and then I just forgot about it( I have now relocated the booklet on one of the highest and dustiest shelves in my library). Now ,perousing BlueMystic´s abstract, I realized that It confirms and supports the theories and facts that Graham Hancock describes in his latest work :"Supernatural". Hancock´s experiences with ayauasca and DMT are nearly identical to Bourrough´s and even the conclusions they come to , if one considers the different mind frame of the two writers and the different experience they were seeking, are remarkably similar.
You can read about "Supernatural" in this section of the forum but if you decide to read it then to begin with "the Yage letters" would probably make some passages much clearer and more belivable.