This is an interesting work, if you're familiar with other Patricia Highsmith novels - and by "interesting," it is that it's not technically a crime novel (i.e., it's not the major theme of the novel), it's another display of the range of her capabilities, rather; also, that when the crime is committed, it's not from the person from whom we're watching the events through - it's sort of a third-person crime, in this way. And not for the usual reason. (I'll leave it there so that, even though another reviewer has told you who the killer is, the novel hasn't been completely blown for you.)
"People Who Knock on the Door" is still a very readable novel, since the differences don't really detract from the reading - it has the same storytelling style of other Highsmith novels, and is not a labor to read for it.
If you're looking for a DEEP WATER/THIS SWEET SICKNESS-esque suburban psychopath tale, you may find it slow and ultimately disappointing. (It is, however, rather like EDITH'S DIARY.)
But if you aren't, read on!