The May 2008 issue of Ellery Queen has this nifty short story. If you've read some of Mr. Block's novels you may know that Keller normally works killing humans. As this story begins, however, Dot (who seems to work pretty much as Keller's agent) explains that he's been hired to kill a dog. The money isn't great, but then the risks aren't so big either.
I can't say much about who hires him without spoiling the plot, but I will say that by the time it's all over, it isn't just Fluffy the pit bull that needs killing.
The best part of the story as far as I'm concerned was the dialogue between Keller and Dot which kind of provided a running commentary on the twists in the plot (of which there are plenty). I've never read one of the novels, so I don't know if it is common for the series.
A very enjoyable read.