In this case, the publisher link below points to Project Gutenberg, which has free downloads, in multiple formats, for this fiction which is long in the public domain.
The story is set in the mid 18th century starting in for the first sections, but becomes a sea-faring story in the Caribbean. The narrator (with the exception of two chapters), Jim Hawkins, is a boy or young man (the book doesn't make his age clear), though it is made clear that he is not grown to the size of a man, and lacks the strength of the adults around him.
Since this story has been in the public domain for many years, I found that every major turn of the story was predictable as I read it, as the story in whole and in parts has been used many times in many, many other stories. That said, the world building is outstanding. The topography, flora and fauna of Treasure Island was very carefully described making the island truly feel like a real place. One quirk of this book, which authors typically try to avoid, is that there are three characters named Tom, and three named John, which I sometimes found to be a little confusing.
Overall, I'm very glad to have read this book, even though the story itself was familiar from other sources. This book is the original origin of that now clichéd pirate with one leg and a parrot, and treasure maps with the treasure trove marked on the map. It is a wonderfully told story and the language was wholly accessible.
Recommended for those that want a swashbuckling nostalgia trip ... due to the familiarity of story ... told in vivid details. Skip it if a pirate fantasy just doesn't sound interesting. Also skippable for those readers that cannot forgive the clichéd tropes (even knowing that this is the book where those clichés were fresh and new).
Project Gutenberg (Originally: Cassell and Company)
Released: 14 November 1883
E-book: 289 Pages