The Woman in White by Wilkie Collins
From Wikipedia: "The Woman in White is an epistolary novel written by Wilkie Collins in 1859, serialized in 1859–1860, and first published in book form in 1860. It is considered to be among the first mystery novels and is widely regarded as one of the first (and finest) in the genre of 'sensation novels'.
The story is considered an early example of detective fiction with the hero, Walter Hartright, employing many of the sleuthing techniques of later private detectives. The use of multiple narratives draws on Collins's legal training and as he points out in his Preamble: 'the story here presented will be told by more than one pen, as the story of an offence against the laws is told in Court by more than one witness'."
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