There’s a great New Yorker article about the book here.
I liked it more than Strangers on a Train but less than most her later work. (I read The Cry of the Owl recently and recommend it).
It is very romantic. Also, the best bit is towards the end when Highsmith basically invents a new genre: The Gay Romance Thriller.
There’s a lot of sociology and psychology in this book but it’s all in the background. Lesser writers would be tempted to have their characters articulate it, and give impassioned speeches or have the villains proclaim their malevolence. But Highsmith’s characters are just ordinary people trapped by social constructs they can’t even see, let alone denounce.