For a reasonable summary of the book, you can browse the article he wrote for The Atlantic Monthly on the same subject back in 2002. I'm going to focus on Chapter 10, which effectively debunks the popular myth that North American in particular was a "pristine wilderness" when the Europeans arrived.
Two months ago I reviewed and excerpted Paradise Found: Nature in America at the Time of Discovery (University of Chicago Press, 2009), which detailed what Europeans found in North America:
"The woods abound with acorns for feeding hogs and with venison. There is considerable fish in the rivers, good tillage land; here is, especially, free coming and going, without fear of the naked natives of the country..." Magnificent chestnut oaks, which these turkeys had wisely chosen as a roost, often rose sixty feet before there were any branches. So, while it’s easy to be impressed by the great tracts of forest carpeting the ridges and valleys of the Appalachians today, we should remember that these forests are nothing like the precolonial forests. Those first explorers found themselves walking through a natural cathedral whose green roof arched fifty or more feet above their heads…”Mann's point in 1491 is that this was not even remotely a "pristine" environment unmodified by humans. Instead, what the awe-struck Europeans found was a world that had been managed for millennia by Native Americas:
A principal tool was fire, used to keep down underbrush and create the open, grassy conditions favorable for game. Rather than domesticating animals for meat, Indians retooled whole ecosystems to grow bumper crops of elk, deer, and bison. The first white settlers in Ohio found forests as open as English parks—they could drive carriages through the woods. Along the Hudson River the annual fall burning lit up the banks for miles on end; so flashy was the show that the Dutch in New Amsterdam boated upriver to goggle at the blaze like children at fireworks. In North America, Indian torches had their biggest impact on the Midwestern prairie, much or most of which was created and maintained by fire. Millennia of exuberant burning shaped the plains into vast buffalo farms...The role of Native Americans was unappreciated because their numbers had been decimated (literally) by epidemic diseases such as smallpox and typhus.
The tricky part of this explanation is not the facts per se, but the implications - especially re tropical rain forests. Amazonia was similarly "managed" by natives to such an extent that at one time it was capable of supporting huge populations, farming manufactured soil. If that argument is loosely applied, it can be used to support the claim that modern man can "improve" the jungles of the Amazonian region through the use of technology to make them more useful and productive.
I'll stop here. Those who have read this book are encouraged to offer comments on this post.