Daniel Defoe, Robinson Crusoe (22.214.171.124): Robinson Crusoe is often cited as the first English novel. It was written in 1719, and was still very popular as Collins was writing The Moonstone in 1868, although it was generally considered to be a children's book. See the "Character Analysis" section for Gabriel Betteredge for more on the repeated references to Robinson Crusoe.
Religious tracts (126.96.36.199): Miss Clack is always ready to hand out religious tracts. The specific titles she mentions are fictional, but they're typical of a certain genre of evangelical religious writings at the time. Wilkie Collins was repeatedly harassed by a group of evangelicals trying to persuade him to repent.
The Storming of Seringapatam (Prologue): The storming of Seringapatam (also spelled Srirangapatna) was a real, historical event, and it's one of the most important in the history of British India. In 1799, British soldiers stormed the city of Seringapatam, which was the capital of Mysore in southern India. After this, the British East India Company (a private, English business that employed hundreds of British people in India) became the dominant power in southern India. The East India Company had a charter from the British government to rule over India, which was a British colony. The British soldiers looted and stole an incredible amount of treasure in Seringapatam. Accurate figures are impossible to come by, but we're talking treasure worth millions.
James Tod, Travels in Western India (1839) (Epilogue, Chapter 3): Collins probably uses Tod's book as the source for much of his information about India in Murthwaite's final narrative in the Epilogue.