What is the significance of the book found wrapped in the oilskin packet? chapter 6

Squire Trelawney is the one who figures out that it's an account book: it has all of the captain's wages from when he first started as a pirate to much larger sums now.The sealed paper includes something much more promising: a map of an island with three crosses of red ink, two on the northern part and one in the southwest. Next to the southwest cross it says: "Bulk of treasure here"