Which 1868 novel is often considered the first full length English detective novel?

"The Moonstone" (1868) by Wilkie Collins is a 19th-century British epistolary novel, generally considered the first full length detective novel in the English language. The story was originally serialized in Charles Dickens' magazine "All the Year Round". "The Moonstone" and "The Woman in White" are widely considered Wilkie Collins' best novels. Besides creating many of the ground rules of the detective novel, "The Moonstone" also reflected Collins' enlightened social attitudes in his treatment of the servants in the novel. Collins adapted "The Moonstone" for the stage in 1877, but the production was performed for only two months.

The book follows Rachel Verinder, a young English woman who inherits a large Indian diamond on her eighteenth birthday. It is a legacy from her uncle, a corrupt British army officer who served in India. The diamond is of great religious significance as well as being extremely valuable, and three Hindu priests have dedicated their lives to recovering it. Rachel's eighteenth birthday is celebrated with a large party, at which the guests include her cousin Franklin Blake. She wears the Moonstone on her dress that evening for all to see. Later that night, the diamond is stolen from Rachel's bedroom, and a period of turmoil ensues. Told by a series of narratives from some of the main characters, the complex plot traces the subsequent efforts to explain the theft, identify the thief, trace the stone and recover it.