If you were going to loan one Terry Pratchett book from the Discworld series to someone, which book would you use?
This is a very often-debated subject… You'll want to tailor the choice to that person's taste.
Some novels have the same main characters as earlier ones, and they make more sense if you've read the earlier ones. There's a reading order guide on L-Space, with a dependency graph by Krzysztof Kietzman (also in picture form). Some of these dependencies are stronger than others, for example Wyrd Sisters and Witches Abroad make sense on their own, but Lords and Ladies is really a sequel to Wyrd Sisters. The Watch novels are really a series, I don't recommend starting in the middle (so, not The Fifth Elephant).
- I'm partial towards The Colour of Magic (the first book in the series) myself, but I know a lot of people don't like it. It's a good point of entry for habitual fantasy readers (The Colour of Magic starts with a Leiber) spoof; Light Fantastic is a direct sequel.
- Wyrd Sisters is a good introduction for a Shakespeare fan.
- Soul Music for a rock-and-roll fan.
- Witches Abroad is built on fairy tales.
- Small Gods is good for someone who likes philosophy of religions.
- Mort introduces Death, and Guards! Guards! introduces Vetinari (the ruler of Ankh-Morpork); both are endearing Pratchett characters.
- Monstrous Regiment is good for a feminist.
- Going Postal is set in Ankh-Morpork, but you don't need to have read the previous books to appreciate it; it shows the city confronted with modernity, 19th-century style.
- Night Watch shows an earlier Ankh-Morpork in a revolution; it's almost independent from the other Watch books.
For a younger reader---especially but not exclusively a young lady---consider Wee Free Men, the first of the Tiffany Achings novels.
My niece is sufficiently mannerly that receiving a thank you note was taken in course, but we heard through the grape-vine that she asked for the sequel, so I think it was a hit.
I would always recommend Feet of Clay, I know its in the one of the watchmen series, but I find it's one of his most rounded books. It has a lot of the running jokes, that are funny when you encounter them in the different books, but stand very well on it's own.
I would also always recommend that you reread it after you've read a few of his other books.
I wouldn't start with the Colour of Magic, as I think that is better enjoyed by someone who's read a few of his books already.
The Truth is the first one I read. It's after Pratchett's mainstays like the Watch or Death have already been developed but those books don't have to be read beforehand to understand who they are in relevance to the story. It's very good at letting you get a taste of the series without being forced to jump into a whole sub-series.
Start with the Rincewind cycle, continue with the early "stand-alones" (small gods. pyramids) and DEATH cycle, then launch into the Watch cycle, then finish off with the witches... but at least do the cycles chronologically, since it adds emotional connection with the characters. My suggestion is to start where he did, even though RIncewind overall is a somewhat weak character (the other university people are much more fun in the later stories), by the time you get to the best parts of the Discworld saga (namely the city watch) you will know all about the (disc)world. It's also fun to watch Pratchett develop his style from spoofing and light-hearted parody to the more bitter cynical humor of his later books. Oh, and Vetinari
and don't forget Strata!