The Uncollected Stories of Patricia Highsmith
The book is divided into two parts: early writings and later writings. It was really interested to see Highsmith's style develop over the course of so many years. While there's a disturbing psychological element to all of her short stories, the later ones introduce a much darker side.
I'm a huge fan of the Tom Ripley series and felt like I could see Highsmith experimenting with ideas in short story form that would eventually be included in Ripley.