Another sort-of-kinda mystery book by WIlkie Collins. It's been made into moves over and over again, the last one in 1948. Wikipedia:
Walter Hartright, a young art teacher, meets a mysterious and distressed woman dressed in white. He helps her on her way, but later learns that she has escaped from an asylum. Next day, he travels to Limmeridge House in Cumberland, having been hired as a drawing master on the recommendation of his friend, Pesca, an Italian language master.
The Limmeridge household comprises the invalid Frederick Fairlie, and Walter's students: Laura Fairlie, Mr Fairlie's niece, and Marian Halcombe, her devoted half-sister. Walter realises that Laura bears an astonishing resemblance to the woman in white, who is known to the household and whose name is Anne Catherick. The mentally disabled Anne had lived near Limmeridge as a child and was devoted to Laura's mother, who first dressed her in white.
And so on. And so on. The Woman in White is later described as"A young woman said to be of disordered wits."
PLUS FAMOUS OPERA! Hooray for heroic dog, though. As long as it's not an opera dog.