Since Robert Louis Stevenson was just starting out when he published Treasure Island, the editor of the magazine Young Folks suggested that he publish his work under the pseudonym "Captain George North." After Stevenson's colossal success with The Strange Case of Dr. Jekyll and Mr. Hyde five years later, in 1886, he got to print his novels under his own name (source).
Captain Flint's Treasure Island, also known as Skeleton Island, may be a fictional place, but there is a real Treasure Island in the United States. It's in San Francisco Bay – not far from Alcatraz – and it used to house a military base. Apparently the island was built using mud from the floor of the bay. People thought this mud might contain gold, hence the name Treasure Island (source).
If you haven't realized it already, Robert Louis Stevenson was quite the free thinker. He wrote a fairly scandalous set of papers against tightly repressive Victorian English sexual morality called Virginibus Puerisque. It's pretty lofty in tone, so you might find it hard to get into after a page-turner like Treasure Island, but it does give us more insight into Stevenson's character and values.