They come in groups, in a way, but they also refer back any which way, anyway. I recommend just the order they were written, it's worked well so far. (about half way through, I think)

Guards! Guards! is a great one to start with. It follows Samuel Vimes, captain of the Night Watch (police force) of the city-state of Ankh-Morpork (loosely based on London) as he deals with a dragon the size of a house showing up in his city and demanding gold. It was summoned by a small group of people with dreams of becoming the shadow government, using a book stolen from the library of Ankh-Morpork's finest (and only) wizarding university. The spell allows them to summon a dragon, directly control all of its actions, and dismiss it at will. Their plan is a cinch: summon the dragon, have it eat a few people, terrorize the city a bit, then find some young upstart with something resembling royal blood and who knows how to flourish a sword and have have him volunteer to fight it. Put on a good show, dismiss the dragon at just the right moment to make it look like he killed it, and watch as the city celebrates and crowns him king. Then all that's left is to puppeteer him from the shadows to rule the city. Unfortunately for the Elucidated Brethren, as they call themselves, the only party less thrilled about this than Ankh-Morpork's existing shadow government is the dragon itself, who doesn't take kindly to being summoned and even less kindly to being controlled. It doesn't take it long to slip their shackles.

It's now up to Sam Vimes and his ragtag crew of "watchmen" who run the other way when they see trouble to solve the case and find a way to get the dragon back where it came from before the whole city goes up in smoke.

Going Postal is also good. It follows conman's-conman Moist von Lipvig as he wakes up in a very comfortable chair the morning after he was hanged, still rubbing his neck, sitting face-to-face with Lord Vetinari, Ankh-Morpork's despotic ruler. Vetinari explains that he sees potential in Moist, so he paid the hangman not to kill him all the way, and is now offering him a better use for his talents: that of being Postmaster of the city's derelict Post Office. Should he refuse he is more than welcome to reenact what a crowd full of people will swear they saw happen to him. After mulling it over, he takes the job, and arrives at the Post Office to find the place full top to bottom with undelivered letters. He can hardly walk through the hallways. Its only two occupants are Junior Postman Groat, who could be Moist's grandpa, and Stanley, who, while the word "autistic" doesn't appear anywhere in the book, absolutely is. He knows everything there is to know about pins ("Last year the combined workshops (or “pinneries”) of Ankh-Morpork turned out twenty-seven million, eight hundred and eighty thousand, nine hundred and seventy-eight pins,’ said Stanley, staring into a pin-filled private universe. ‘That includes wax-headed, steels, brassers, silver-headed (and full silver), extra large, machine- and hand-made, reflexed and novelty, but not lapel pins which should not be grouped with the true pins at all since they are technically known as “sports” or “blazons”, sir") and when he gets upset he has what the book calls "one of his Little Moments" (which are never actually described). As a person on the spectrum myself, I have to hand it to Pratchett -- the portrayal is exaggerated a bit, but all things considered not inaccurate (especially compared to some... ahem other portrayals of autism in the media that we've seen lately that I could mention. I will never forgive Sia for making the movie Music.) Sadly Stanley is very much a minor character. Anyway.

After the advent of the Clacks system (semaphore towers that claim to "send messages at the speed of light" -- think telegraphs, but in a universe without electricity), the post office didn't see much use, so it downsized. Mail just sort of piled up since there weren't enough people to deliver it and throwing it away was illegal. Sleeping in amongst the mail, Moist swears he can hear the letters whispering their contents to him. He has visions of the post office in its heyday. This place is old, and it wants to return to its former glory.

They're both very good books and Pratchett absolutely deserves his reputation as a British humorist who, as one newspaper put it, "wants us to feel and think as well as laugh." Both these books have a lot to say on the subject of government and they say it in the best way possible. Can't recommend enough.

Based on some random reddit thread. Its a good book to start i think. Just go for it