John Milton, Paradise Lost. (2.1). "Finding in the lowest depth a deeper still" echoes the line in Paradise Lost IV, 75-8 where Satan says, "Which way I fly is Hell; myself am Hell; / And in the lowest deep a lower deep/ Still threat’ning to devour me opens wide,/ To which the Hell I suffer seems a Heav’n."
18th-century novels (15.1). The digression with which Dickens opens this chapter is obviously in imitation of eighteenth-century novelists, including Lawrence Sterne in Tristram Shandy, and Henry Fielding in Tom Jones and Joseph Andrews.
"Murderous melodramas" (17.1) Plays about crime were enormously popular at this period. Even Oliver Twist, which makes fun of the popularity of bloody melodramatic plays, was adapted into at least eleven different theatrical versions before the end of the nineteenth century alone.
William Shakespeare, Macbeth. (18.1). "The crying sin of ingratitude" is an allusion to Duncan’s line in Macbeth, 1.4.14-16.
Newgate Calendar (20.17; 3.6.35). The Newgate Calendar was a collection of eighteenth-century criminal biographies. Some moralists worried that biographies of famous criminals might glamorize crime and inspire young people to turn to crime, like Charley in 3.6. Oliver’s reaction in 1.20 is the reaction Dickens thinks any civilized person should have—he thrusts the book away in disgust.
The Bible, Revelation 20:12-13. (38.101). After dropping the locket and ring into the river, Monks says, "If the sea ever gives up its dead—as books say it will." This is a reference to the book of Revelation in the New Testament.