Tony Horwitz wrote this book after visiting Plymouth, MA and seeing the rather disappointing Plymouth Rock. Then he realized he didn't have a very good idea of what happened in the century between Columbus's "discovering" American in 1492 and the Pilgrims' landing in 1620.
As it turns out, a lot of people explored America, both before and after Columbus, who never actually set foot on North America, and the before the Pilgrims' arrival - mainly Spanish conquistadors. The Spanish explored a large portion of the North American continent, including the Pacific coast, the Grand Canyon and the Oklahoma prairies.
Having grown up in Florida, where the conquistadors are part of local lore, I already knew a lot about them, just like I knew that St. Augustine, FL is the oldest, continuously settled European city in the US.
In the book, Horwitz follows the trails of many of the explorers through the US, looking for signs of their presence. He even travels to the Dominican Republic, where Columbus first landed, hoping to get a glimpse of the supposed remains of the explorer.
It's a fascinating journey, both through our history and our country - one well worth reading.