The Inner Life of Martin Frost — Paul Auster

Searching for solitude, the writer Martin Frost borrows a friend's country house. Waking up one morning, he is shocked to find a nearly naked young woman beside him in bed. She also has a key to the house and claims to be the owner's niece. Martin's initial annoyance at Claire's intrusion is rapidly forgotten as he falls passionately in love with her. Even when it is revealed that Claire is not who she claims to be, their idyllic passion continues--until she suddenly falls ill.