I've just started re-discovering Pratchett's books, in a fairly random order, depending on what I fancy at the time, and indeed on what I spot in charity shops. In the last couple of months, I've read and enjoyed paperback copies of Going Postal and The Truth, and have read Interesting Times on my Kindle. I'm just coming to the end of Raising Steam, also Kindle version.
I don't think I'm losing out by reading the books out of chronological order, but it might have lessened my enjoyment if I'd started Raising Steam, for instance, if I'd not encountered Moist von Lipwig before, or not known about "clacks". As it is, I've had a thoroughly enjoyable few evenings catching up on the work of the dear departed Pterry.
I have to say that, for a "mere" writer of humorous fantasy, TP's books are really very well crafted. Everything hangs together (and I say that with extra admiration, as I realise that he was still writing while his Alzheimers was fairly advanced), and the action slides seamlessly from comedy to drama and back again. That said, Raising Steam is probably the least funny of the books that I've read - I don't mean that in a bad way, though - and the death-count, described or implied, is very high for a Discworld novel!
I'm loving the little details - even though I've still only read about 10 of the Discworld series, I'm spotting the in-jokes, returning minor characters, and the cameos from characters seemingly from other works (for instance, two of the baddies in The Truth will be very familiar if you've seen PulpFiction, and there's a family straight out of The Railway Children in Raising Steam!).
I found it the most heartbreaking (although I still haven't read the Shepherd's Crown, so I still have a new Discworld novel to look forward to...) It was clearly his own requiem to his creation, the obvious ending of an era: magic replaced by technology. And yet, as with so much of his stuff, there is way more to it than that. He evokes the magic, the life, that animated steam technology. The end of an era marks the start of a new one, after all. The discworld abides. And now I am crying again.