The Woman in White by Wilkie Collins. 
Possibly Wilkie Collins' most famous novel, The Woman In White remade the Gothic Horror novel by taking its characters and tropes and setting them in commonplace surroundings among "people like us", Featuring unforgettable characters such as the incomparable Count Fosco and the redoubtable Marion Halcombe (a woman for whom male Victorian readers politely inquired of Wilkie the address as they wanted to marry her), The Woman In White with its compassionate treatment of those suffering mental distress ought to be credited with having put the cause of mental health care a hundred years ahead - had not Jane Eyre with its madwoman in the attic been generally credited with the reverse. Read it for the female doubling central plot device alone - a rare feature in the writing of men about women.