This tidy novel by John D. MacDonald would probably be lost today in the tidal wave of 20th-century thrillers and mysteries, if it weren't for the 1962 movie version with Robert Mitchum and Gregory Peck (and the 1991 remake by Martin Scorsese, which I have not seen). While the book has a bit less action than the movie, it does develop the characters in much more interesting ways. The Sam of the book, for example, is more human and less steely jawed than Gregory Peck's portrayal, while his wife has more of a take-charge attitude than Polly Bergen displayed. We also learn more about the villain than the 1962 movie revealed.
The plot and various details of the setting are quite different from the 1962 movie. The setting is not specific (and has nothing to do with the Cape Fear River). The climax and the events leading up to it are especially unlike the movie (a nice surprise).
One other aspect of the book I need to comment on: MacDonald's portrayal of law practice is a little off the mark. First, in a conversation with one of his law partners, Sam is told that he's the token honest man in the firm, that every law firm has one, and that most lawyers are continually bending the rules. In reality, most lawyers are very particular about the rules; the rule-benders and corner-cutters are the minority. Second, at one point Sam calls his partner to "ask" if he can take a week off. Law-firm partners (especially in a small firm like Sam's) are all "bosses"; they don't need to ask each other for permission to take time off. (They may ask another partner if s/he wouldn't mind covering a matter while they're gone, but that's not what Sam was doing.) In any event, those are minor quibbles.