I love Terry Pratchett dearly, and I like to spread the joy around. However, he's written around 50 books, so I think it's a good idea to give newcomers a bit of background on the books, otherwise it seems rather daunting. The majority of Pratchett's books take place in a world called Discworld, which is flat and circular, and rests on the back of four elephants, which stand on the back of A'Tuin, a giant turtle. I believe he wrote the books pretty much in chronological order, but within the whole Discworld series there are smaller story arcs, or sub-series, that follow a specific main character. I'll give you lists of books grouped together in their own story arcs, and give you suggestions on where to start. However, I haven't even read half of the books yet, so some of the books I list I won't have much to say about, since I haven't read them yet. But, regardless, I'm sure they're awesome, because Pratchett is a genius. So, without further ado, here's my list!
Tiffany Aching -- This arc follows a young girl named Tiffany Aching as she grows up and learns to be a witch. Magic has a large role in Discworld, but Discworld witchcraft is very much like traditional, historical notions of witchcraft, and involves much more being sensible and doing the things that need to be done than wand-waving and magic spells. These were written to be young adult books, but I think they still have much to offer adults. I think you'll find some interesting stuff about gender and how ideology/stories shape our understanding of the world. I think you'd enjoy these, and they're not a bad place to start getting into Discworld.
1. The Wee Free Men -- Tiffany must rescue her brother and the baron's son from the queen of the fairies, armed with good sense and a frying pan, and with the help of the six-inch tall, blue Scottish men called the Nac Mac Feegles.
2. Hat Full of Sky -- Tiffany, now studying with older witches away from home, finds herself possessed by an ancient creature who compels her to do dreadful things. She must defeat it before it destroys everything and everyone she cares about, and before it destroys her too.
3. Wintersmith -- Tiffany finds herself tangled up in the ancient story of the changing of the seasons, and is confronted with her first romance. Except, her would-be suitor is the spirit of the winter, who wants eternal winter and Tiffany for his bride.
4. I Shall Wear Midnight -- ill tidings roll across the Disc, as suddenly people begin to fear and hate witches. Tiffany must defeat a mysterious and soulless apparition to save herself and all of witch-kind.
City Watch -- these books primarily follow Sam Vimes, the captain and eventual commander of the Ankh-Morpork (a large, main city, and the setting for a lot of Discworld books) city watch. These are books for adults, unlike the Tiffany Aching books, and deal with the heavier topics, like racism/race relations, imperialism, violence, justice, and the law. Vimes is a quintessential noir anti-hero, and the other characters who are part of the watch are diverse and interesting. These are some of my favorite books in the series, and I think this is also a good place to start reading Pratchett.
1. Guards! Guards! -- A plot to overthrow the patrician of Ankh-Morpork goes horribly wrong, and suddenly a giant dragon is ravaging the city. Captain Vimes of the night watch must pull himself out of the gutter (quite literally) and whip into shape his defunct night watch to catch the conspirators and defeat the dragon.
2. Men at Arms -- A dangerous and deadly new weapon finds its way into the wrong hands. The body count rises as the weapon urges its owner to continue killing. Vimes must catch the culprit and prevent even more murders.
3. Feet of Clay -- Strange crimes are committed that seem to be linked to the golems (clay men animated by mystic words placed inside their heads) of the city. As the plot plays out, the status of golems as property comes into question, and one begins to wonder if they are alive or not. Meanwhile, the patrician is being poisoned, and Vimes must root out yet another plot aimed to bring down the ruler of the city.
4. Jingo -- A mysterious island rises from the sea exactly half way between Ankh-Morpork and Klatch. The two countries seem to be gearing up for a war, but Vimes senses that foul play is afoot. Vimes is dead-set against a war, and will arrest entire armies if he has to in order to stop it.
5. The Fifth Elephant -- Vimes is sent as a diplomat to the country of Uberwald, where the Low King of the dwarves is about to be crowned. However, an important artifact has been stolen, and Vimes is on the case, determined to root out the culprits before the throne crumbles under the Low King and chaos (along with disturbed commerce between Uberwald and Ankh-Morpork) ensues.
6. Night Watch -- Vimes finds himself thrown back in time to a period of revolution in Ankh-Morpork. He becomes drawn into the events, and has to save the day, along with teaching his younger self how to be a good copper. This book has interesting similarities (and dissimilarities) to Les Miserables.
7. Thud! -- Tensions between the dwarves and trolls of Ankh-Morpork rise as the anniversary of a historical bloody battle between the races approaches. A suspicious murder takes place, and while Vimes is on the case, a mysterious, quasi-demonic entity seeks out Vimes and attempts to use him as an agent of revenge. Vimes must solve the crime and overcome his own inner darkness at the same time.
8. Snuff -- Vimes finally takes a vacation to his wife's country estate. But as soon as he gets there, his old copper instincts kick in and he knows something isn't right. A sordid plot unfolds involving goblins, murder, and high-speed riverboat chases!
Moist Von Lipwig -- These books follow a character with a rather unfortunate name. He's a con man and a classic trickster, but is set on, if not the correct path, then on a path that is most useful to the patrician of Ankh-Morpork. These books have some anti-corporate themes, and the second book has interesting things to say about money and capitalism.
1. Going Postal -- Moist Von Lipwig is to be executed for his white-collar crimes, but finds himself instead being offered the job of rehabilitating the defunct post office, which is no task for the faint of heart. However, as he does so, he finds powerful enemies in the large business of semaphore tower (basically visual morse code using squares of black and white) business.
2. Making Money -- Moist grows bored with the post office, which now runs smoothly. So, to keep him busy and sharp, the patrician of Ankh-Morpork gives him the task of overhauling the currency system. Boldly, he begins the transition from the gold standard to paper money -- but not without making plenty of enemies along the way.
3. Raising Taxes -- I've not read this book yet, but I'm sure it's just as good as the two before it.
Death/Susan Sto Helit -- A common character in the Discworld is Death. As in, black robe, skeletal Death with scythe. He also happens to adopt a daughter, and that daughter has a daughter, named Susan. Susan manages to "inherit" some of her adopted grandfather's talents, much to her chagrin, since because Death is constantly having existential crises, she is often called upon to fill his role. Also, interesting fact about Death: he appears in every Discworld novel except The Wee Free Men.
1. Mort -- Death hires an apprentice, Mort, who falls in love with Death's adopted daughter and begins to meddle in things he really ought not to meddle in -- with humorous results, of course.
2. Reaper Man -- Death retires and experiences something he's never experienced before: life. Meanwhile, strange things are afoot in Ankh-Morpork, where nothing seems to be dying like it should. This book has interesting themes about industrialization/urbanization.
3. Soul Music -- This book is about sex, drugs, and rock and roll! Well, it's about rock and roll, but one of three isn't too bad. Death has an existential crisis and abandons his post, pulling an unsuspecting Susan in to cover for him. Meanwhile, this strange new form of music is taking the country by storm.
4. Hogfather -- An assassin is hired to kill the Hogfather (the Discworld equivalent of Santa Claus), and through a twisty plan, he begins to succeed. However, as Death tells Susan, this means that tomorrow the sun won't rise. Susan must investigate and stop the mad assassin from ending the world.
5. Thief of Time -- A clockmaker is hired by a mysterious lady to build the perfect clock. What he doesn't know is that this will stop time forever and destroy history. Susan must work together with a young man who is more than he seems, and may be a little bit like her (i.e., not quite human).
Stand Alone Novels -- some books are not part of a story arc and can be read on their own.
1. The Truth -- The printing press is invented, and with it, the newspaper. William de Worde -- a young man who is good with words and is devoted to the Truth -- stumbles across an insidious plot to "un-elect" the patrician of Ankh-Morpork. He is determined, with a little bit of investigative journalism, to uncover the truth.
2. Small Gods -- Brutha, a slow and thoughtful young man, finds himself the prophet of his god, Om. However, he finds Om in the shape of a tortoise, and lacking much of his power. Brutha must re-instill people's faith in Om (for, at this point, the people believe in the institution of the Omnian church, and not Om himself), and stop the Omnian church from going down the wrong path of torture, war, and inquisition before it's too late.
3. Monstrous Regiment -- A twist on the classic "Mulan story." A woman dresses as a man to join the army so she can find her missing brother. She soon finds out, however, that the regiment she joins is entirely made up of women dressed as men.
4. Moving Pictures -- I haven't read this one, but I do know it's about movies.
5. The Amazing Maurice and His Educated Rodents -- alas, another one I haven't read.
6. Pyramids -- another one that I have not, unfortunately, read. It's about ancient "Egyptian" culture, as far as I know.
Witches -- this arc follows a group of witches and their various adventures in the world. I've only read a couple of them, so I'll mostly just list them for you.
1. Equal Rites -- A wizard is born, but the only problem is that she's a girl. There have never been any female wizards before, and as she enters the Unseen University (the wizard university in Ankh-Morpork), trouble is bound to arise.
2. Wyrd Sisters -- A rather humorous parody of Macbeth.
3. Witches Abroad -- Esme Weatherwax is called abroad and takes her sister witches, Magrat Garlick and Gytha Ogg, to defeat the mysterious, evil fairy godmother in the far-off country of Genua.
4. Lords and Ladies
5. Maskerade -- I hear this one draws a lot from Phantom of the Opera.
6. Carpe Jugulum -- deals with vampires
Wizards -- this arc follows a particularly inept wizard named Rincewind. That's all I can really say about it, since I haven't read any of these books. I know that some of them are his very early works, and I've heard the opinion that they're not as good as his later works. But I've also heard that some people really like them. I really couldn't say myself. I'll just have to read them.
1. The Color of Magic
2. The Light Fantastic
5. Interesting Times
6. The Last Continent
7. The Last Hero
8. Unseen Academicals
Pratchett and Neil Gaiman -- These two authors with very different styles join forces to write a hilarious book. It's not a Discworld novel, but still definitely worth a look.
Good Omens -- The time of the coming of the Antichrist is upon us, much to the dismay of demon Crowley and Angel Azraphael, who have both come to enjoy their lives on Earth. The two join forces to subvert the plans of both God and Satan, with, of course, hilarious results.
Well, that's all of them that I know of. This link is to a visual representation of the reading order, but it doesn't have all the books as it's not completely up-to-date. I may also be missing some of the books, but if you really want to know more, a quick Google search should find you some more information. His Wikipedia page is good, and there is actually a Wiki just for Discworld.