Dark Secret by Marina Anderson

Harriet Radcliffe is bored with her life. At twenty-three, her steady job and safe engagement seem very dull. If she is to inject a little excitement into her life, she realises, now is the time to do it.
But the excitement lying in wait for Harriet is beyond even her wildest ambitions. Answering a job advertisement to assist a world-famous actress, Harriet finds herself plunged into an intense and secret world of sexual obsession - playing an unwitting part in a very private drama, but discovering in the process more about her own desires than she had ever dreamed possible...