I'm going to spoil the ending of this one to some degree. I'll try to say some general things first, so you can tell whether you care about the spoiling.
This is another novel reissued as a British Library Crime Classic—just this past year, 2023. Margot Bennett was the first woman to win the Gold Dagger (though there was at least one woman on the shortlist every previous year). She never wrote another crime novel after this one, mostly turning instead to writing for television. Did she quit on a high?
In some ways, yes. The book is a fairly successful marriage of the classic form of the whodunnit with some then-contemporary psychological content. As in the previous winner, the psychologies of the main characters are foregrounded and examined as putative explanations for their actions—murder is more than base motive. But, unlike with the previous winner, there is a genuine, well-executed mystery at the core of the novel: which among four suspects, all complete shits of varying consistency, murdered the protagonist's friend?
The protagonist is a youngish woman, Nancy, who is a writer for magazines. Her dead friend is Sarah. The four suspects are all terrible, terrible men with whom Nancy, Sarah, or both have romantic pasts or presents. I say "romantic". This feels like a euphemism here, since none of the relationships described are obviously loving, in the sense we should hope to use that word.
The book works, insofar as it does, because the four male suspects are all convincingly bad. One of them is actively trying to frame Nancy for Sarah's death, and going some way to succeeding. She doesn't know which of them it is, and the police are on to her (not without reason—she doesn't always help herself). Meanwhile, all four terrible men take turns at being personally horrible to Nancy, each in their own entirely believable way. So we believe that any of them could be capable of the crime, and we don't know which it is until the last few pages. This engaging, well-wrought plot is written up tightly enough to create tension, with adept use of flashback to fill in the characters. It's all helped along by snappy, funny dialogue; the introduction by Martin Edwards speculates that Bennett identified with Nancy, and she certainly gets all the very best lines.
There are two reasons why I'm not entirely sold on this book. The first is that it is so appallingly, irredeemably squalid. Perhaps this is exactly as it was for young women writers in the 1950s, but oh dear, the grime and the stink and the horrible, horrible men make you itch as you read. This is an achievement, in a way, but not a pleasant one. The second reason is more serious, and involves the spoiler. After the mystery is solved, after all is settled, after we know which of the shits is a murderer, after we and Nancy know they are all shits: after all that, she falls swooning into the arms of the worst of the shits, the one who has treated her most appallingly, whose only positive quality appears to be his innocence of murder.
Now, there is a question of how seriously this ending should be taken. It's possible that Bennett meant it as a sort of tragic coda—that she sees the obvious problems with the proposition, and sees Nancy as deserving of them, as no better than she ought to be. But I don't think this is so. I think this is meant to be a consummate, cheering resolution. To modern sensibilities, it is certainly not cheering, and really not believable—in fact, close to incomprehensible. It mars what is otherwise a very fine book, worth reading for its plot, its dialogue, and its itchy squalor.