If you are unacquainted with Rankin's bizarre books, this may well be a good start. It's not his best book, but it's his first. Part of the fun reading Rankin is in the repetitions: Rankin quotes himself constantly, so you won't miss any of that when you start off from here.
"The Antipope" is the first novel in the Brentford series, in which an ordinary London suburb is the scene of grotesque battles between Good and Evil. It's up to Jim Pooley and John Omally, two bums with an insatiable appetite for beer, to save the world, with the help of a mysterious professor and some other highly improbable characters. In this book, the adversary is an evil tramp posessed with formidable powers, who is about to take control of the world as the Pope of some dark new Church.
Two things distinguish Robert Rankin from other comic SF/Fantasy writers like Douglas Adams and Terry Pratchett: his profound knowledge of the occult, and the sheer outlandishness and pomp of his stories. Rankin calls himself a tall-tale teller and his books far-fetched fiction. Some scenes in this particular book, like the disastrous cowboy night, and the vain attempts to open a mysterious parcel, just project themselves before your eyes, as if you were watching a movie. And make you laugh aloud.