This is, quite simply, one of the most important books I've read in years. It's the kind of book that you'd recommend to your friends that aren't necessarily environmental. They may not be going in, but I'd be surprised if they didn't have a different viewpoint after reading the book.
The central concept of The World Without Us is what would happen to the planet if, in an instant, all humans disappeared. As Weisman says in the Acknowledgments of the book:
"It was a deceptively simple question that ... lets us view our Earth's current myriad stresses from the disarming vantage of a fantasy in which we supposedly no longer exist, yet somehow we get to watch what unfolds net. Watch, and maybe learn ..."
Through exhaustive research and interviews with experts in many fields: architecture, geology, biology, zoology, archaeology, etc., Weisman has created a unique thought experiment that shows how destructive we've been (and will continue to be beyond our demise), but also how quickly the earth would eat up signs of civilization. He goes all over the planet - downtown New York (with scenes reminiscent of I am Legend), the oil fields of Texas, the cradle of life in Africa, etc.
It's an idea that is not that far-fetched. Seemingly successful cultures in our past have mysteriously disappeared:
"When you examine societies just as self-confident as ours that unraveled and were eventually swallowed by the jungle, you see that the balance between ecology and society is exquisitely delicate. If something throws that off it can all end. Two thousand years later, someone will be squinting over the fragments, trying to find out what went wrong." -- Archaeologist Arthur Demarest
It's a fascinating, yet scary, book that packs in a bunch of science but not so much that the lay person couldn't understand it. And it's so well-written that it reads more like fiction than dry history. If you want to give someone a different perspective on the ways in which we affect our planet without beating them over the head with environmentalism, recommend this book.