Pure, distilled essence of Parker. This didn't displace the Engineer trilogy in my affections, but it's absolutely where I'd suggest anyone curious about Parker start. The novella format really plays to her strengths, I think; it's long enough that she can build up layers of irony, but short enough that readers unhappy with the lack of sympathetic protagonists in her full-length works may be able to read it without frustration.
My only quibble is that sometimes things were a bit too
clearly spelled out, particularly in some of the longer, latter letters. The twist was pretty easy to see coming, too, but then, with Parker, you know the rules going in: the worst will always happen. It's how she writes it that makes it worth reading.