After reading Diggers, I couldn't find the third book in the Bromeliad, but it turned up the other week, so of course I read it. I must say, I didn't start it with much enthusiasm. The end of Diggers makes it clear how Wings concludes, as this book runs in parallel, and I don't find his writing for children as engaging as the Discworld novels (or at least the ones for adults - I've yet to move onto the young adult ones, which will probably happen when I run out of the rest).
So, I trudged through a good proportion of the book, but I must say it caught my imagination. A few twists managed to appear in what could have been an extremely linear story (with a known ending), and it did keep me engaged. While "Truckers" and "Diggers" hint at a single vehicle, Wings is well-named, involving at least 4 kinds of flying transport mechanisms! (I hope that's not too much of a spoiler. :)
I think this sounds like weak praise, but... well maybe it is. It's entertaining, but not riveting, I don't begrudge the time I spent on it, but I've read so many books by him. Ah well.