This is the third installment of the Charlie Numbers series, Charlie and his Whiz Kids friends find themselves in a mystery and adventure when a mammoth tusk they have discovered leads them to take on an international criminal who smuggles elephant ivory. The opening scene, where Charlie escapes the villain's henchmen by jumping off a boat into freezing water, had me hooked. The rest of the story leading up to it was also good, with interesting characters, lots of action, and cool scientific facts. This can be read alone, but the references to Charlie's previous adventures will make readers eager to read the whole series.