Uncharted is just a short little Novella that MUST be read after On the Island. It's the story of "Bones" and a man named Owen.
This was a great little story, that did draw me in right off the bat. You get little snippets from Anna and TJ but not a lot. It really is a separate, but connected, story. As it went it did slow down a wee bit. Not as engrossing as On the Island, but still really enjoyable.
Everything is wrapped up nicely, and it does tie up other loose ends that you might have had after reading On the Island.