It's a first-rate story of suspense, a verdict with which estimable fellow bloggers such as Kate Jackson, Xavier Lechard, and John Norris (all of whose blogs I strongly recommend in the unlikely event you're unfamiliar with them) concur. I agree with the view that in effect it amounts to a gender reversal on the traditional woman-in-jeopardy trope. This time the protagonist in jeopardy is a man, the naive but likeable David Marks. He is a school teacher who has taken up cab driving in the evening as an odd sort of psychological therapy following the death of his wife in a hit and run accident.
One night he is asked by a beautiful young blonde woman to take her to a house in Connecticut. She offers him eighty dollars for the return trip. One of the problems with writing about sums of money in a novel is that inflation dates the narrative, but in 1975, this was a huge sum of money for such a journey. David is puzzled and becomes suspicious, especially when his passenger asks him to take her on another such journey a few days later.
Before long, David finds himself embroiled in a murder plot. This is a novel of mounting suspense, although it also has an excellent plot twist. Fletcher's taut writing reveals her apprenticeship as a radio writer: she knows how to make each word count. The book was turned into a film in 1982, and renamed Hit and Run (such a forgettable title that it's been re-used several times for other films), alternatively known as Revenge Squad. I haven't seen the film, and judging by comments on the internet, it's barely worth watching. But the book is definitely worth reading.