The 1990 400 page "Guidebook to Zen and the Art of Motorcycle Maintenance" by DiSanto and Steele includes lots of fascinating background to the book: maps, chronology, western philosophy, eastern mysticism, critical reception, correspondence with Robert Redford on the film Redford intended to make of the book, works about ZMM, etc etc.. Worth a look if you find a cheap s/h copy on Amazon.
ZMM must have been reprinted within the last couple of years because I found a new copy in one of the few remaining real bookshops in the area.
Some of Pirsig's other novels are quite haunting too.
Bryn