This is a combination of a fairly middle of the pack story mixed with a host of interesting details and historical data about the very early beginning era of exploration. We see the story begin in the belly of an Arabic galley in which our hero is a slave chained to an oar, rowing until his death. He manages to get free, get home, establish his identity, and then the real troubles start.
There's a romance, betrayals, political struggles, duels, and all sorts of events which fill the book but overall its mostly about the early age of exploration where Europeans tried to map the world using the best instruments and science at their disposal. The hero, Andre Bianco of Venice, ends up with the Portuguese court and he has a secret from the Arabs called Al Kamal. With this, he can determine latitude which has been to this point a mystery to sailors.
It is quite entertaining, and Slaughter is a terrific, largely forgotten author. The story has pretty much every element of adventure fiction from the time packed into a quick-moving, well-told tale with plenty of lush description. However, the protagonist is too good. He's smarter, stronger, tougher, more capable, more skilled than everyone.
Andre fights better than the warriors, is stronger than the strongest men, is smarter than all the other mapmakers and scientists. He's such a paragon that you never really have any sense of his danger once he's properly introduced. And that diminishes the drama and really my ability to connect to the story.
But the real star of this book is the sometimes too-dense historical information, jammed into the story about how navigation, sailing, shipmaking, and fighting was done at sea at the time. Five decades before Columbus sailed to find a western route to the Indies, things were pretty unclear and primitive, but developing very quickly. And that saves the story from otherwise being a pretty forgettable Marty Stu adventure.