One novel on the list:
Zen and the Art of Motorcycle Maintenance, by Robert M PirsigRead about the other entries on Mullan's list.
Philosophical disquisition or autobiographical novel? However you define this once-cult classic, no literary motorbiking column could omit it. The narrator introduces you to the main schools of western philosophy via metaphors from motorcycle mechanics, while travelling across the US with his son on the back of his bike.
Zen and the Art of Motorcycle Maintenance also appears on Sebastian Beaumont's top ten list of books about psychological journeys.