"Treasure Island" by Robert Louis Stevenson
Treasure Island was first published in 1883. It was the first full-length novel that Stevenson wrote, and preceded The Strange Case of Doctor Jekyll and Mr. Hyde by three years.
In a small English village, Jim Hawkins helps his mother run an inn. One day a pirate by the name of Bill takes on lodgings in this inn, and after Bill's untimely demise young Hawkins discovers a treasure map among his belongings. Hawkins and his two guardians then fit out a ship to search for the treasure, though subsequent events reveal mutinous intentions within their hired crew.
Treasure Island is a good, easy read, and has captivated children for generations. It's not as deep as Stevenson's Strange Case, but it's concise, well-paced, and never grows boring.