Having read most of Highsmith's books and being an inveterate reader of books about misfits and sociopaths, I was completely taken aback by my reaction to this. I felt so close to the protagonist that, although I kept Highsmith's books at the time, after a while I had to make this one leave home. It made me way too uneasy to have it close by.
I was having a slightly difficult life when I read it. I expect now it wouldn't scare me to death. Still, my feeling is that this is not a book to read if you are in the depths of despair. Be advised.