How many books about Robin Crusoe did Daniel Defoe write?

Explanation

Rubinson Crusoe is the title character of Daniel Defoe's novel "The Life and Strange Surprizing Adventures of Robinson Crusoe, of York, Mariner, written by himself" and "The Farther Adventures of Robinson Crusoe" (1719), and the by now almost forgotten "The Serious Reflections of Rubinson Crusoe" (1720).