Unchallenging, easy-reading Dan Brown-a-like without much excitement. An antique dealer gets hold of a journal from someone who sailed on a British Navy ship, which describes a journey in which he uncovers secrets among the ship's gentry, including the fact that they are carrying a piece of wood that came from the cross of Christ. So the adventurers travel to Jamaica to find the buried treasure.
It transpires that a criminal gang is following them, and MI6 are following them. They get captured, escape, re-captured, and then just as they are about to be killed, the secret police jump in. Yee-hah.
The book's only saving grace really is that half of it is a transcription of the ancient journal, which gives an interesting account (and story in its own right) of sea-faring in the age of pirates.