Diggers is something of a mid-trilogy book. It's The Empire Strikes Back for the Nomes. Interestingly, it appears the three books aren't chronological - it looks like this book ties up with the end of Wings (I'm guessing). The same themes appear in this book as in Truckers, although, well, it feels like not so much goes on.
Whereas Truckers dealt with how people react to change, Diggers is about what happens in a power vacuum, when tough times appear. To that degree, it's pretty optimistic, since the religious nutter who takes over doesn't actually get voted in, but just takes power for himself. Oh, and the problem of said nome is solved a little too quickly, and a little too pat for my liking. I guess it is a children's book after all.
I enjoyed it. Not as much as Truckers, but it's an adequate, if mildly aimless, sequel. Now I have to find out if we can locate our copy of Wings!