Robert Anton Wilson's most famous book is ILLUMINATUS!, written with Robert Shea. It has been available in several different editions.
It was originally published as three separate novels, but since 1984 has been sold in an omnibus trade paperback.
When the books were first published, Wilson and Shea wrote a synopsis each for the second and third volumes.
The synopses were omitted when the omnibus edition came out, which I always thought was a shame because they contained bits not found elsewhere in text -- for example, a clear explanation of the self-destructing mynah birds, birds taught to say "Here, kitty kitty kitty!" and turned loose in New York City to freak out anyone who might be watching.
I raised the matter with Shea the one time I met him, and he told me he and Wilson were unable to convince the publishers the synopses were little literary works in themselves.
ILLUMINATUS! also is available as a series of audiobooks, performed by multiple readers.