River can totally read Usopp's mind. He has no mental powers or barriers at all. And you may talk about it! He'll probably be freaked out, though, when he figures out that you can actually read his mind.

As said before, Usopp won't notice as he has no mental powers. As for what's in there, there's ADVENTURE, fear, worry, doubt and a boatload of stories. Usopp has a pretty active imagination, both good and bad.