Poison in the Pen — Patricia Wentworth

Lovers meet, kiss--and someone is going to tell. A poison pen is busy scribbling billets-doux in rustic Tilling Green ... until murder becomes more effective than malice. One lethal letter leads to a body in Manor House lake. At the house, heiress Valentine Grey receives shocking news with her wedding presents. Nasty notes may turn her wedding party into a wake--especially if the one person who can identify the village's secret sender ends up murdered.

MAUD SILVER, PRIVATE ENQUIRY AGENT
She is everyone's favorite spinster-detective. From her Edwardian hair style to her beaded shoes, she is the very model of a governess which indeed she once was. Now she has taken on the much more challenging occupation of private enquiry agent. Armed with stubborn British common sense and an iron will to succeed, she is one of the best at tracking murder, as memorable as the finest creations of Agatha Christie.