The mystery event in Pokemon Sapphire is the "Devon Corporation Incident", where Team Aqua and Team Magma are both attempting to control the legendary Pokemon Groudon and Kyogre.
To solve this event, you need to:
* Beat the game and reach the post-game.
* Talk to Steven Stone in Slateport City.
* Obtain the Devon Goods from the old lady in Slateport City.
* Travel to the Seafloor Cavern and defeat Team Aqua/Magma.
This is a story-driven event and does not involve finding an e-reader.
Perhaps you're thinking of a different Pokemon game or event?