Jacobs is able to show that the real problem with cities isn't overpopulation - rather, it is exactly the opposite! The major problem with cities today is that they aren't dense enough. Empty sidewalks are inviting only to criminals. Children, shop keepers, and families hate an empty sidewalk.
Furthermore, city planners compound the situation by moving businesses (and therefore commerce) away from residences - thus resulting in a further decline of sidewalk traffic.
If you're going to be involved in city government, planning, or land use, you should definitely read this book. I'm a small government conservative, and lots of other conservatives are scared by Jacobs -- but let me tell you -- this is the future of America. We should accept and embrace this urban challenge.