I don't think it's possible to not know the story of Treasure Island if you live in an English speaking country. I remember watching the old Disney version when we had a substitute teacher in elementary school, and who doesn't love the Muppet version :) My exposure to this story has been long and engrained, but I never realized how many changes all of these movies made until now.
Treasure Island tells the exciting story of Jim Hawkins, a young English boy who ends up on a pirate-ridden adventure by complete coincidence. The Captain, a "seafaring man" staying at his family's inn, dies leaving behind the debt of his bill and a group of dangerous men who have promised to return later that night. Jim and his mother go through The Captain's trunk to collect the money he owes them and the stumble on a paper packet that turns out to be the only map to the location of the Captain Flint's treasure. Flint's treasure was a myth that even those who weren't connected with seamen knew about, so Jim takes the map to his friend the local doctor and thus begins a voyage to claim the treasure. They employ a man named John Silver to be the ship's cook for the voyage, they trust him completely. Unbeknownst to Jim and his group, the crew Silver finds for them are all Flint's former men and they plan to take the gold for themselves once they find it. Jim learns of their treachery and his group of honest men escape to the island before the pirates can do anything. There are several battles that take place, several life or death situations, treasure, marooning, and everything other typical pirate related excitement you can think of.
I can see how sensational this story was in the 1800s when this book was first written, as well as why it is still greatly endeared today. I now can point out all the things every movie has left out (example: do you know who Ben Gunn is?) as well as how many little things they changed that completely altered the outcome of the story. Even though I have heard this story most of my life in one way or another, it was like reading a different tale in a way. I got a real feel for just how sly John Silver is as well as how stupid Squire Trelawny is for trusting him. No matter how many films you've seen, you still stand to learn something more and enjoy the book they are all based off of. This is s story that will no doubt be a favorite as long as there are people around to read it :)