The introduction by Minette Walters is a real bonus with this edition. She picks up on the themes of casual racism, and female homosexuality that run through the book. I don't share her optimistic view that Sayers "...leaves us with little sense that she had any sympathy for the prevailing prejudice against blacks..."
Her attitude to lesbian relationships is far more interesting. In my experience of popular middle brow fiction from the twenties through to the fifties there's a surprising amount of acceptance for women in same sex relationships, it's not an acceptance extended towards men. It's also noticible that these women are generally a bit older, and I'm guessing that this tolerance is based on the lack of men to go around. These relationships are essentially unthreatening to the status quo.
Its noticible here for example that an older pair of women who had lived together, bred horses, and made a tidy fortune out of it are tacitly approved of, but when a character in her 30's looks to be encouraging the affections of a girl in her 20's it's frowned upon (though to be fair the older woman in that scenario is the main suspect, so it is an undesirable attachment). Nonetheless it's an interesting insight into what might and might not have been considered okay.
As for the plot - this is a much more enjoyable book than 'Five Red Herrings' to read, and reminded me why I used to like Sayers so much. Lord Peter hasn't quite yet evolved into the perfect man and is arguably better for his imperfections. The plot is less concerned by who, more with how, the how is ingenious and convincing (apparently it might not work, but the theory is sound). Miss Climpson is introduced and is rather marvellous, the body count is extravagant which adds to the tension, and there are lots of references to books I know - which is always fun.