Apparently he claims that everyone came to the America's before Columbus did...Celts, Hebrews, Egyptians etc etc. He has no evidence from what I understand, just some half hearted belief that academic archeologists are hamstrung by being forced to feed into the "party line" or lose their jobs. And that the whole Columbus discovered America is just another form of racism.
Columbus discovered the Americas. This sentence seems to get would-be academics keyed up. "No he didn't, there were already people here."
Of course there were people here. Of course they weren't standing around confused about who and where they were, to suddenly become enlightened when the dude jumps off the Nina. I think we can all take the statement for what it is worth, Columbus is the first human to discover America in an age when the discovery changed the migratory patterns of the Western World. The Vikings got here, but it didn't have an impact on both continents. In fact it had no impact on the world at all. If the Celts came here they suffered the same fate. If the Egyptians did they suffered it as well.
But when Columbus did, it changed the course of human history.
Here's a review of the book. The reviewer hammers Zimmerman for having no evidence, as there is no evidence for pre-Columbian contact with the Americas, aside from the Vikings for a very short period.