Sarah Challis 
Killing Helen [EPUB ebook] 

Starting an affair with Robert is easy. Bored with the awkward, boorish single men of her own age and flattered by the confident attentions of a handsome stranger, Harriet is excited by the idea of a lustful friendship, as Robert chooses to describe it. A commitment-free fling is ideal, and the existence of a wife is a minor inconvenience. But then Harriet finds out about his other mistress…