Okay the question may not be very clear as it is hard to explain this book in brief.
Basically it involves a schizophrenic person who is constantly seeing a little thing/person/demon (maybe a leprechaun; I don't remember) who is kind of like a twisted version of the Great Gazoo
He then stumbles upon a conspiracy based around the legend of Robert Johnson's deal with the Devil. It extended to include a lost 30th song recorded by Johnson before his death and something to do with the devil's note.
It also showed that Elvis was alive at the time (early 2000s) by means of a symbiotic alien/bug.
It was set in England if that helps.
You'd think that with all the weirdness I'd remember the name of the book.