Artistic sense of humour
So that's it. The fact that all three titles refer to an artifact is no more than a coincidence, though it does make a nice pattern. Before I'd finished the third one, the artist Eric Rohmann, who drew the wonderful covers the books had in their first Knopf editions, asked what the third book would be called, and before I could tell him, volunteered THE SOPHISTICATED MONKEY-WRENCH.
I just finished the first book – other than the abrupt ending, it's pretty good. I was searching the 'net to find out why the name of the first book was different in the US (and the movie).
I'm a few pages into the second book now. No spoilers, please!