Odd that you would put Moving Pictures in the same category as The Truth. The former is about a new technology/social change starting to take hold in Discworld, but by the end, the Magic Reset Button is pressed and the Disc goes back to being the way it was (Soul Music does the same thing). The latter, though, was one of the first books where Pratchett started letting the Disc actually evolve: Changes happen, and they stay changed, at least until the next change comes along and builds on top of them.
Well, yes and no. The Truth represents the start of the technological evolution. The political evolution starts with Wyrd Sisters and Guards! Guards! Ankh-Morpork politics change drastically between G!G! and say Thud. Even more drastic change if you allow the events of Night Watch to be folded into the mix.
Ditto the Witches series. Relationships and power evolve. The only books that seem to hit the reset button in general are the Death and Rincewind books.
Eh, the Witches books feature plenty of personal growth, but then, even the Rincewind books have that (there’s no way that the Rincewind of The Color of Magic would have volunteered for the mission in The Last Hero, for instance). But the Witches books still feature very little evolution for the world as a whole. Yes, yes, in Wyrd Sisters, the witches break their rule about not interfering with politics… but one gets the impression that breaking their own rules has always been a part of what witches do. And they leave Lancre with the same basic system of government as it’s always had, just with a new guy on the throne.
If you read the Tiffany Aching series, then after reading Wintersmith you should listen to this CD of music specifically recorded for the book.
Another book that’s a stand alone is The Amazing Maurice and his Educated Rodents, which I absolutely adore.
I would go with either the Witches series or the Vimes series… hard for me to guarantee one over the other. I can’t decide between Sam Vimes or Granny Weatherwax as my favorite fictional character of all time.
Hogfather is a great book too, especially if you enjoyed the philosophical commentary in Small Gods… it has one of my favorite philosophical conversations ever put on paper.
This is very similar to the suggestion put forward by the Quirmian philosopher Ventre, who said, “Possibly the gods exist, and possibly they do not. So why not believe in them in any case? If it’s all true you’ll go to a lovely place when you die, and if it isn’t then you’ve lost nothing, right?” When he died he woke up in a circle of gods holding nasty-looking sticks and one of them said, “We’re going to show you what we think of Mr Clever Dick in these parts…”
Hogfather was the first really Discworld-y book I read (before that was Pyramids, but that takes place in its own bubble, so to speak). It’s a great way to get acquainted with the wizards (especially poor old [young] Ponder . . .) as well as see Death as more of a sympathetic figure. Not to mention lots of philosophical musings. If you want to save that for Christmas when you’re feeling more seasonal, *Going Postal * is a great introduction to Vetinari, the politics of Ankh-Morpork, etc.
Apart from those two, I’ll echo Guards, Guards and The Wyrd Sisters as good places to start with the Watch and the witches, respectively. If you find yourself not liking Guards Guards, I started the Watch books rather haphazardly with The Fifth Elephant, and loved them; there’s nothing wrong with just picking up whatever looks most interesting to you and jumping in. (I also first met the witches with Witches Abroad, which comes with an intro from Pterry basically telling you “don’t read this one first!” . . . and it was great).
Once you have the backstory on Ankh Morpork politics, The Truth is a real treat and probably my favorite Discworld. Just puttin’ it out there. 
I started with Feet of Clay, just because I needed something to read and I remembered Michael Dirda, the Washington Post book reviewer, had liked it. It was awesome.
I’d tried reading Discword before, but I’d started with The Light Fantastic, which isn’t where to start. I read two pages and gave up.
That’s what I’ve been doing. I’m currently halfway through the series, having gotten as far as Hogfather, but I’m taking my time–don’t want to use them all up too quickly. (Even though they are addictive and I’ve read a number of them back-to-back.)
*Pyramids *is my least favorite of the series so far, and even then I loved the beginning part, about the Assassins’ Guild, where it actually *felt *like Discworld. I’m just not too crazy about it when Pratchett too closely parallels specific real world cultures or phenomena, as in *Pyramids’ *ancient-Egypt-with-a-funny-name. I had a bit of the same issue with Moving Pictures, although the explanation of how Discworld motion picture cameras work is priceless. My favorite characters are the Witches and Death.
Is it the one about believing little lies as practice for believing the big ones?
I treasure my copy, signed with a “Ho! Ho! Ho!” Always glad I got the chance to meet and actually chat to him.
Note that the Tiffany Aching books and The Amazing Maurice and his Educated Rodents are young adult books. They are written for young teens and such. I strongly advise against reading those until you’ve read all the regular Discworld books as you can handle.
(In fact, avoid the Tiffany Aching books completely. They are incredibly bad IMO. I don’t see how kids can stand them, let alone adults.)
Don’t listen to ftg, Mahaloth. The Tiffany Aching books are awesome.
ftg knows not whereof they speak. The Tiffany Aching books are stellar.
My signed first-edition Pratchetts are among my most prized possessions.
More Tiffany Aching love here.
I’m here to support Tiffany Aching too.
And his other books too, some of which approach (and even achieve) literature, most of which are great, and a few are, well, still a lot better than I’ve ever written.
I was in Waterstones the other day and spotted that they’ve made a graphic novel out of Small Gods. I’ll have to go back and buy it.
But what I really want is a Complete Works. Leather-bound hardback, of course, on onion-skin paper.
And see - I can’t understand how any one gives a shit about Harry Potter after reading Tiffany Aching. It’s like the difference between bubblegum-flavored wine coolers and a shot of pure Glenfiddich.
Which I guess is another way of saying that I don’t understand why children like Tiffany Aching, either, but the ones that do are something special.
But to answer the question - I always read book series in chronological order according to the book’s timeline (not necessarily publication order.) I can’t do it any other way.
You should read Equal Rites, because it will be relevant when you get to I Shall Wear Midnight. But its sequel, Wyrd Sisters, is the first book where Pratchett really hits the ignition stage.
Equal Rites should be read, but not until you have read most of the Witch series. It’s really a rough draft for the Witches, and is better understood after familiarity with the coven. I also think that the Witch series should be read before the Tiffany Aching books. You really have to understand Esme for the Tiffany books to have maximum impact. That requires Witches Abroad and Lords and Ladies.