Patricia Wentworth 
The Girl in the Cellar [EPUB ebook] 

A witness who remembers nothing is in mortal danger.A young woman regains consciousness and finds herself on some cellar steps. At the bottom of the steps there is the corpse of a dead girl. She cannot remember who she is, what has happened or why she is there. Terrified and confused she manages to find a way out and as she flees she runs into Miss Silver, who offers to help her. A letter in her bag is the only clue to her identity. But by investigating what has happened to her will she find herself in danger? Can she trust the letter writer? And who is the girl in the cellar?