Not everyone enjoys epistolary novels, which inevitably spend a lot of time on things that are important to the characters in the moment they write letters to each other, but not to the overarching plot that they're unaware of. To me, though, that's part of their charm, and one of the great strengths of the epistolary style is the development of character, voice, and viewpoint that it enables, or even demands.
I will say that the discussion of books, thinkers and ideas current in the late 1920s (the book is set in 1928-30 and was published in 1930) was less interesting to me, both because I've not read the books or, in many cases, heard of the people, and because the ideas, vitally interesting at the time, are now nearly a century out of date, and have either become commonplace or, in most cases, been dismissed as inaccurate and unhelpful. One character is writing a biography of a 19th-century mathematician at the start of the book, and finding the research into the Victorian way of thinking heavy going, and that's also how the passages of 1920s thinking felt to me. At the same time, I'm sure I'm missing nuance because of not being immersed in the contemporary zeitgeist and the books and ideas. At one point, for example, a recent play by Somerset Maugham is mentioned, and when I looked it up on Wikipedia, the plot has close parallels with what's happening in this book at the time.
The letter-writers are a wonderfully mixed bunch. There's an outwardly cynical and modern poet/novelist (the one writing the biography). In his letters to his fiancée, far from wanting a superficial free-love relationship of the kind he probably depicted in his work, he reveals his complete devotion to her and his worry that their relationship will be damaged by the jealousy and other faults he observes in the Harrisons, a couple who live downstairs from his flat. There's the paid companion of Mrs Harrison, a woman in poor mental health. There's Mrs Harrison herself, a self-involved and self-justifying woman who is always breaking down in tears because her husband treats her rudely and unkindly, even though to the novelist he conveys the impression that he loves her deeply and just lacks the ability to express it. There's Mr Harrison, too, writing genially to his son and giving an impression at complete variance with what the others are observing. A large part of the strength of the book is that it portrays the utterly different perspectives these people have on the same events and people, including each other. And the voices are just right for who they are. The novelist is prolix and worries too much about abstractions; Mrs Harrison is silly and emotional; Harrison's engineer son from a previous marriage is pragmatic, systematic, and has no time for the novelist's meanderings. They read like completely different people are writing their letters, and that's hard to do.
The novelist's flatmate is a painter, who paints a portrait of Mrs Harrison that gives away more than either of them realize. At the halfway point of the book, the painter is holidaying in a shack in a remote part of England with Mr Harrison, who has kept up a friendship with him even though he no longer lives in the flat in the same building. The painter goes up to London for some business and convinces his friend the novelist to come back to the shack with him, and when they arrive, they find Harrison dead, poisoned by mushrooms - on which he was an expert. Events leading up to that point have made the painter a clear suspect for murdering Harrison, inasmuch as he has a strong motive. But did he have means? Opportunity? And even if he did, can anybody prove it?
The solution is clever (as was the crime), and we get the full emotional impact of it because we see it through the eyes of the novelist, who's the best among the cast at talking about his feelings. Overall, a fine piece of work, and though it isn't a traditional mystery novel, it's something better than that in many ways. Sayers' books were always at their best when she wasn't writing a conventional mystery anyway.