Terry Pratchett is a hilarious British author who writes novels set in a mostly fantasy setting that parody society, other novels, and random things like Shakespeare, organized religion, and the press.
They can be read mostly in any order, HOWEVER! There are kind of "groups" that the books (27 or 28 so far) can be put into:
Death and Death's family
The Witches
The Watch
Rincewind
And then there's a few books without a grouping.
Of those, Death and The Watch are the two best. Anyway, in each of these groups, there is a certain order in which to read the books:
Death etc:
Mort
Reaper Man
Soul Music
Hogfather
Thief of Time
The Witches:
Equal Rites
Wyrd Sisters
Witches Abroad
Lords and Ladies
Maskerade
Carpe Jugulum
The Watch: (my personal favorite)
Guards! Guards!
Men at Arms (my favorite of the favorites)
Feet of Clay
Jingo
The Fifth Elephant
Night Watch
Rincewind: (least favorite)
The Color of Magic
The Light Fantastic
Sourcery
Interesting Times
The Last Continent
The Last Hero
Non-group books:
Pyramids
Eric
Moving Pictures
Small Gods
The Truth
Monstrous Regiment (my favorite out of ALL of his books)
The best books out of all Pratchett's wonderful stories:
Men at Arms
Small Gods
Maskerade
Thief of Time
Night Watch
Monstrous Regiment
About Monstrous Regiment, my personal all-time favorite:
It's the story of a girl who dresses up as a man joins the army of the 'underdog' country, Borogravia, who's soldiers are fighting for a Duchess who may or may not actually be alive. The story's strengths are in it's wonderful characters - Maladict, a vampire addicted to coffee; Sergeant Jackrum, a really fat guy who's freaking awesome and IS my friend Paul, it's scary..... Paul's fat and everything too... O_O *shivers*; Lietenant Blouse, the girliest man among the manliest women.......; and so and and so forth. It's wonderful, and the ending ... well, it should have ended differently let's just say. I'm rewriting it, because I belive such a wonderful book should not end that way.... the joke just went a bit too far.
In summation - Terry Pratchett is a parodist, funny and wonderful, but in the true tradition of satire, manages to make lasting comments about society and life and people in general.
An excerpt from Small Gods (because Monstrous Regiment is, regretfully, at a friends house

):
"... it's more to do with how people live."
"What, lolling around all day while slaves do the real work? Take it from me, whenever you see a bunch of buggers puttering around talking about truth and beauty and the best way of attacking Ethics, you can bet your sandals it's because dozens of other poor buggers and doing all the real work around the place while the fellow are living like-"
"-gods?" said Brutha.
There was a terrible silence.
"I was going to say kings," said the Great God Om, reproachfully.
"They sound a bit like gods."
"Kings," said Om emphatically.
"Why do people need gods?" Brutha persisted.
"Oh, you've GOT to have gods," said Om, in a hearty, no-nonsense voice.
"But it's GODS that need PEOPLE," said Brutha. "To to the believing. You said."
Om hesitated. "Well, okay," he said. "But people have got to believe in something. Yes? I mean, why else does it thunder?"
"Thunder," saud Brutha, his eyes glazing slightly, "I don't -
" - is caused by clouds banging together; after the lightning stroke, there is a hole in the air, and thus the sound is engendered by the clouds rushing in to fill the hole and colliding, in accordance with strict cumulodynamic principles."
"Your voice goes all funny when you're quoting," said Om. "Anyway, that's just an explanation. It's not a REASON."
"My grandmother said thunder was caused by the Great God Om taking his sandals off," said Brutha. "She was in a funny mood that day. Nearly smiled."
"Methaphorically accurate," said Om. "But I never did the thundering. Demarcation, see. Bloody got-a-big-hammer Blind Io up on Nob Hill does all the thundering."
"I thought you said there were hundreds of thunder gods," said Brutha.
"Yeah. And he's all of 'em. Rationalization. A couple of tribes join up, they've both got thunder gods, right? And the gods run together - you know how amoebas split?"
"No."
"Well, it's like that, only the other way."
"I still don't get how one god can be a hundred thunder gods. They all look different..."
"False noses."
"What?"
"And different voices. I happen to know Io's got seventy different hammers. Not common knowledge, that. And it's just the same with mother goddesses. There's only one of 'em. he ust got a lot of wigs and of course it's amazing what you can do with a padded bra."
There was absolute silence in the desert. The stars, smeared slightly by the high-altitude moisture, were tiny, motionless, rosettes.
Away toward what the Church called the Top Pole, and which Brutha was coming to think of as the Hub, the sky flickered.
Brutha put Om down in the sand.
Absolute silence.
Nothing for miles, except what he had brought with him. This must have been how the prophets felt, when they went into the desert to find... whatever it was they found, and talk to... whoever they talked to.
He heard Om, slightly peevish, say: "People've got to believe in something. It might as well be gods. What else is there?"