Henry Wade 
No Friendly Drop [EPUB ebook] 

At first it seems that Lord Henry Grayle has taken an overdose of sleeping medicine, but the autopsy reveals a tiny amount of scopolamine along with the draught—harmless in itself, but fatal when mixed …

A poisoner with apparently expert knowledge is at work in the great house at Tassart. But from what motive, and how? Before he can find an answer to these questions, Detective-Inspector John Poole is faced with a second, more horrible murder.