Hands up any reader whose post is failing to arrive! The chances are, it has disappeared into Discworld where, oddly, it isn’t arriving either. In fact, nobody in the city of Ankh Morpork has received a letter for decades, as a once-glorious system collapsed into ineptitude, leaving letters and parcels to back up like blocked lavatories. We are all going postal in Britain — that useful American term signifying lunacy — but only Terry Pratchett can make us laugh about it.

Pratchett, contrary to what his detractors say, doesn’t offer escapism. His world, increasingly subtle and thoughtful, has become as allegorical and satirical as a painting by Bosch. Death makes a frequent personal appearance, torture is common and corruption absolutely rife. The hero of Going Postal, Moist Lipvig, is no super-man but a conman, forger and thief. Condemned to death, he is saved in the first chapter by the saturnine Lord Vetinari in return for working as Postmaster General. Doing the obvious thing — running away — isn’t an option, because Lord Vetinari has given him a golem, Pump, which will tirelessly hunt him down and force him to keep at it.

Moist is a delightful rogue. He simpers and smirks and knows every one of his faults, and is determined to keep on sinning. He invents Discworld’s first stamps, intending to pull off a magnificent fraud; he delivers letters with the intention of absconding with them. Then he discovers that, as Pump solemnly puts it, “every undelivered message is a piece of space-time that lacks another end . . . They communicate and change the nature of events. When there’s enough of them, they distort the universe around them.”

The morbidly playful universe of Discworld (supported on the backs of four elephants who are in turn supported by a giant tortoise) might seem in no need of distortion. Magic and quantum physics have already made it a crazy place: the challenge for Pratchett’s creations is to bring about order and justice while incidentally discovering their own capacity for good. Children laugh at the comedy of liars being caught out, but adults will recognise the resolute humanism of his thesis, as well as an admirably subversive streak of political satire that goes all the way back to Swift. Moist plays a more traditional role than Gulliver, and is forced to prove his feelings for his sceptical beloved by solving the murder of her father, who alongside other ghosts now haunts a method of communication called “the clacks”, a cross between the telegraph service and the internet. He wins both her heart and his own freedom.

Yet the plot, though it rattles along, is secondary to the pleasure of the writing. Characters who start off as seemingly boring, eccentric or stereotypical develop loveable quirks and flourishes. Pratchett offers postmodernism for smart kids. He not only turns Dr Frankenstein’s lisping assistant, Igor, into an entire species of medically obsessed ghouls, but does what fantasy writers are not supposed to do, and confounds expectations set up by familiar archetypes. A post-boy, obsessed by pins, blossoms when transferring this Asperger’s-type monomania to stamps. Pump the golem mutates into ponderous pathos and affection. Most surprising of all, Moist embraces his destiny as a hero-postmaster, golden suit, a winged helmet and all, because he can’t give up an agreeable lie. “This was where his soul lived: dancing on an avalanche, making the world up as he went along, reaching into people’s ears and changing their minds.” The trickster tricked is always a captivating spectacle.

Pratchett’s joy in his creations, in jokes, puns, the idea of letters and language itself makes Going Postal one of the best expressions of his unstoppable flow of comic invention. Every once in a while, the anoraks get it right. If only some of them would join the postal service.

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