But does it seem "essential?"
The Robert Anton Wilson fans who run Ultraculture on Wednesday published an article by Maher Zaidi on "The 4 Essential Robert Anton Wilson Books for Blowing Your Mind."
Zaidi's picks are Illuminatus!, Cosmic Trigger 1: The Final Secret of the Illuminati, Prometheus Rising and Quantum Psychology.
This seems to be a reasonable list, but many of us would at least add the Schrödinger's Cat Trilogy. Cosmic Trigger 2: Down to Earth and The Widow's Son seem essential to me. I also like the collections of short pieces, such as Email to the Universe and The Illuminati Papers.
RAW also is included in Ultraculture's article on "8 Celebrities Who Practice Chaos Magick."