Mostly Harmless. I didn't like that one. It was somehow bleak and left me worrying that DNA was in a bad place when he wrote it. I'm going to be a heretic and say that I did like how Colfer continued the series.

In a Discworld novel, an off-hand remark mentions Ponder Stibbons wanting to build a Van-De-Graff-generator by tying cats to a wheel. I wish I could remember which book it was.