he suggested that this may have been a factor in her willingness to take part in the risky William Tell stunt. The Yage Letters After leaving Mexico, Burroughs drifted through South America for several months, seeking out a drug called yagé, which promised to give the user telepathic abilities. A book composed of letters between Burroughs and Ginsberg, The Yage Letters, was published in 1963 by City Lights Books. In 2006, a re-edited version, The Yage Letters Redux, showed that the letters were largely fictionalised from Burroughs' notes. Beginning of literary career Burroughs described Vollmer's death as a pivotal event in his life, and one that provoked his writing by exposing him to the risk of possession by a malevolent entity he called "the Ugly Spirit": I am forced to the appalling conclusion that I would never have become a writer but for Joan's death, and to a realization of the extent to which this event has motivated and formulated my writing. I live with the constant threat of possession, and a constant need to escape from possession, from Control. So the death of Joan brought me in contact with the invader, the Ugly Spirit, and maneuvered me into a life long struggle, in which I have had no choice except to write my way out. As Burroughs makes clear, he meant this reference to "possession" to be taken absolutely literally, stating: "My concept of possession is closer to the medieval model than to modern psychological explanations ... I mean a definite possessing entity." Burroughs' writing was intended as a form of "sorcery", in his own words – to disrupt language via methods such as the cut-up technique, and thus protect himself from possession. Later in life, Burroughs described the Ugly Spirit as "Monopolistic, acquisitive evil. Ugly evil. The ugly American", and took part in a shamanic ceremony with the explicit aim of exorcising the Ugly Spirit. Oliver Harris has questioned Burroughs' claim that Vollmer's death catalysed his writing, highlighting the importance for Queer of Burroughs' traumatic relationship with the boyfriend fictionalized in the story as Eugene Allerton, rather than the shooting of Vollmer. In any case, he had begun to write in 1945. Burroughs and Kerouac collaborated on And the Hippos Were Boiled in Their Tanks, a mystery novel loosely based on the Carr–Kammerer situation and that at the time remained unpublished. Years later, in the documentary What Happened