This book didn't really work for me, though I can imagine that others might not feel the same way. A multiple murderer has escaped from a "secure" psychiatric facility. His continued psychological problems lead to further killings...
So far so bog-standard. Here's the first thing that isn't standard: The book opens from the killer's perspective. I found this to be difficult and unpleasant reading, not because it's horrendously graphic about the murders (it isn't) but because...I don't know, really. It's like the opening section of The Sound and the Fury but with difficult-to-understand swapped for difficult-to-stomach somehow. It's not a great analogy, since Westlake isn't really doing stream of consciousness (thankfully) though we are "treated" to the killer's alarming paranoid thoughts in abundance.
The second thing that isn't standard is that we keep shifting perspective: We don't stay with the killer, we visit an actor and a part-time police chief, too. We aren't given the usual one person who tries to solve the crime to latch on to as some-one to propel us through the narrative. Instead we jump about between these several folks. (The killer's perspective remains unpalatable every time it turns up.) It doesn't really work as the whole book feels disjointed.
Here's a positive thing: the part time police chief investigating two of the murders is portrayed as not competent or at least not confident of his competence, since he is in fact a Professor of English most of the year...this makes him more interesting than many a fictional detective. It's a shame that the entire story wasn't from his perspective, though that would have posed a lot of structural problems.
And a final negative: the very end verges on suggesting that the criminally insane should just be put to death and the psychiatrist who was treating the killer is portrayed as somewhat crazy, certainly misguided, for believing differently. Can't agree with that.