The Grifters is a 1963 novel by Jim Thompson, later adapted by Donald E. Westlake into the 1990 film of the same name.
The film of the book keeps faithfully to the original story, only changing details for time and flow. For example, the character of Carol, the nurse who attends to Roy and becomes his lover, is removed. In the book, Roy is arrested while on vacation, but released; the film omits this, probably because there are enough sub-plots already. Other changes are mostly cosmetic; Roy's real cover job in the book is only a cover in the film, and in the book Roy had been dating Moira/Myra for three years instead of the film's three months. Lilly's full beating from the mob boss is removed from the film, and Moira/Myra doesn't realize that Roy is a grifter until halfway through the film, as opposed to much sooner in the novel.
Probably the biggest change is the above-mentioned sub-plot with Carol, the nurse. A novel allows more space to explore characters and their motivations, while a film is constrained for time. Removing Carol allowed Moira/Myra more screen time and also gave more credibility to Roy's conflicted emotional attraction to her and to Lilly, his mother; in the film, she is the one who prevents him from associating with the nurse, and so is seen to have a controlling influence on his life, even though they have been separated for years.