From Doon with Death by Ruth Rendell (Inspector Wexford #1)

An ordinary life. An extraordinary death.
The trampled grass led to the body of Margaret Parsons.
With no useful clues and a victim known only for her mundane life, Chief Inspector Wexford is baffled until he discovers Margaret's dark secret - a collection of rare books, each inscribed from a secret lover and signed only as 'Doon'.
Who is Doon? And could the answer hold the key to Wexford solving his first case?