What order should I read Terry Pratchett's books?

Is there an order for reading his books

Not really no. Although there is sort of a kind of a timeline, all the books are self sustaining.

As you may have read they are divided in a couple of groups (wizards, witches, greater ahnk, watch, Moist and DEATH) and those I would probably say start with the first ones of each group. Or don't.

Reading the Vimes of night watch and then going to Guards! Guards!, and finding out how much he has changed can be a pleasure on it's own.

Just one thing, the writing style changes ALOT. In the first couple of books the style is much more... looser? until Pterry started lending his world views to his characters which turned a good fantasy series into a great series period.

I would say Thief of Time because it was the first one I read, but honestly any order is fine. Just buy one, start reading it, and then amaze as you suddenly have 23 Discworld books....

(Assuming we're talking about Discworld books here) The order they were released in is the obvious one, and how I did it, but I'd honestly not recommend it. I enjoy the first couple of Rincewind books but I don't think you absolutely have to read them first. Within all the 30+ books set in Discworld there are several groups that get dealt with, there's the Rincewind books, then you have the witches, there are books where Death is, if not the main character exactly certainly is one, and then you have the ones about Moist. There's also the Tiffany Aching Young Adults books. But the ones I recommend as the starting point are always the books about the Watch.

So that means you're going to start off with Guards! Guards!, then look at this guide. Specifically under the header For Readers Who Understand That The Watch Books Really Are The Best

Read Gaurds! Guards! first. Mort is a good book to read second.

I've often seen the following order passed around:

If you've not read any others, the whole Death trilogy (Mort, Reaper Man, Soul Music) are a good place to start. Guards Guards! is also a good starting point too.

I started with Guards Guards and branched out from there, but if I could go back I think I'd want to read them in order. I don't think the roughness of the earlier books would have felt so pronounced for me if I didn't already know how much better he would get 13 years later. I'm not an advocate of reading them as discreet series either, as there are numerous cameos that you'll be completely oblivious to otherwise.