Definitely his best books since Vulcan's Forge, Medusa Stone takes place in the African Nation of Irtrea( I hope I spelled that right), where Philip Mercer is on a Geological expedition to find a rare ore that can produce energy. Obviously not everyone wants him to find it, and that is where the conflict, and action in the story develops.
Medusa Stone moves at a good clip, and you are never bored, but...the only reason I didn't give it five stars is that the characters don't have any personality, particularly Mr. Mercer, who is a cardboard copy of Dirk Pitt, without the sense of humor.
I wish Mr. Du Brul would spend more time developing his character. The only thing you do know about him, is his soft spot for an old drunk named White, that his parents are both dead, and that he traveled a lot as a kid. He puts nothing in the story to make Mr. Mercer a likable character, and that is the main flaw in all his books. If he is going to continue this series, I'd HIGHLY recommend that he make his hero someone we can identify, and sympathize with, and not some cardboard, no-personality action figure.