Robinson Crusoe by Daniel Defoe (1719) — The novel itself. Loosely inspired by Selkirk's marooning but set in the Caribbean, this is one of the founding texts of the English novel. Read it before you go and the island feels stranger and more familiar at once.
Selkirk's Island by Diana Souhami — A careful, modern reconstruction of Selkirk's four years and four months alone, drawing on the actual documents and the geography of the island. Far closer to what really happened than Defoe.
A Cruising Voyage Round the World by Woodes Rogers (1712) — The contemporary account by the privateer captain who picked Selkirk up in 1709, by which time the sailor was running down feral goats barefoot and had nearly forgotten how to speak. Short, vivid, and free in many editions.