This omnibus volume (which I picked up in a local secondhand bookshop) contains Strong Poison, Have His Carcase (so I've doubled up there) and Unnatural Death. Annoyingly, they are presented out of order, so that a significant character featured in Strong Poison is actually introduced in Unnatural Death. This character is Miss Climpson, who is a smart, shrewd, observant, middle-aged woman who is nevertheless capable of appearing to be a mere harmless gossip... I suspect Miss Climpson may have provided the template for Miss Marple? In fact they first appeared in 1927, with Miss Marple making her debut in December of that year. Hmm. I guess we will never know.
Strong Poison is however, most notable for the introduction of Harriet Vane, wrongly accused of murder. I was a bit disappointed that Lord Peter seems to fall for her at first sight rather than gradually coming to appreciate her qualities, which are pretty subdued here -- no wonder, since she's sitting on Death Row for the entire book. In her brief appearances, though, she is clearly both intelligent and the owner of a sense of humour, which bodes well for their future.
Apparently Sayers based some of this book on her own experience of an unhappy love affair with a bohemian author. Must have been so satisfying to kill him off!