Armadale

Armadale is Wilkie Collins' third best-known work after The Moonstone and The Woman in White. When The Younger Son heard I hadn't read it, he loaned me his copy. It held my attention well throughout. It's available to be read online.

from the back of the book:

The Athenaeum reviewer of Armadale (1866) was only one of many contemporary critics horrified by Lydia Gwilt, the bigamist, husband-poisoner and laudanum addict whose intrigues spur the plot of this most sensational of Victorian 'sensation novels'. When Miss Gwilt flings herself from the first-class deck of a Thames steamer, her attempted suicide sets off events that led to Allan Armadale inheriting Thorpe-Ambrose in Norfolk, romantic rivalries, espionage, counter-espionage and greedy plans for murder.

Wilkie Collins drew upon popular newspaper headlines and new technology - particularly the penny post and the telegraph - to lend extra pace and veracity to his brilliantly elaborate and gripping melodrama. T. S. Eliot regarded Armadale as being, after The Woman in White and The Moonstone, "the best of Collins's romances". Modern readers will find the flame-haired Lydia Gwilt refreshingly, if alarmingly, different from the general run of heroines in Victorian fiction.

Wilkie-Collins.info says it is "Collins's longest novel, published in 1866 and dedicated to John Forster."