Having read all of the Alex Delaware books before this one in sequence I wasn't expecting anything out of the ordinary. I have liked all of them (so far), but none of them stood out as great to me.
This one changed that, to me it was the best of the lot, there is a genuine sense of impending tragedy about the whole thing, as the plot creeps forwards.
It's probably because it isn't like the others in that it isn't based around a patient that Delaware is currently treating, instead he appears to be being stalked by a killer. Excellent stuff.