Archer's hired to discover who sent his client's husband a letter accusing her of infidelity. Introduced to the family and friends at a party as a Hollywood agent, he is sensitive to the growing tension and explosive atmosphere. The reader knows of course that somebody's going to be murdered, but these early chapters are among the most skillfully written to build suspense that I've ever read.
Written in 1950, the inclusion of a homosexual couple was quite daring although there is not graphic description, and isn't significant enough a factor of the plot to either offend or attract a reader.
Read this and I'm sure you'll find it on your own list of crime classics.