Well, I have now read the first two Tiffany books, as well as "The Last Hero".
And they are both good, but in a way, I sort of feel that in a way, "The Last Hero" was more of a "for younger readers" book.
Now, mind you, this isn't about the content, but about the style. The books about Tiffany are simply very, very much like the normal witch books. And Pratchett, while a very, very good writer, makes out Tiffany more mature than I suspect of a real 9-year old. But that's something pretty much any writer has a problem with, because frankly, our brains are working so differently when we've finally grown up, we can't really see it through the eyes of a kid anymore. Our memories of what we did back then - well, at least my memories - feel more and more skewed and "adjusted" to that I sort of insert what I thought about the situation (though the actions remain the same).
And the thing about "The Last Hero" was that it felt like it jumped from one place to the other, all the time. It was a good story, but a bit more fragmented. And while the illustrations were marvellous, they also helped breaking up the flow of story, thus making it appear a bit more like a book for younger readers...
But rereading it is pretty nice. That's when you're expecting all the illustrations, and can focus more on the story itself. Which is a typical Pratchett story.