As introductory economics books go, The Undercover Economist isn't a bad choice. Unlike the wildly popular Freakonomics, which is about surprising or unusual applications of economics, The Undercover Economist explores more traditional topics: prices, scarcity, markets, taxes, government influence, and globalization, to name a few. The book covers those concepts more clearly than the typical economics textbook, making it fairly accessible to the layperson (note: my experience with economics textbooks was in 1989, but I have the impression that they haven't changed for the better since). My favorite chapter is the final chapter about China. It describes the disastrous economic policies of Mao Tse-Tung and how Deng Xiaoping reversed them.
Less convincing is the chapter enumerating the glorious benefits of globalization. Although Harford's perspective as an economist made me consider that globalization isn't quite as evil as progressives like me are inclined to believe, his environmental arguments fall flat. For example, Harford naively trusts multinational corporations to install the same pollution controls everywhere regardless of whether the law requires them.
Harford's writing style is okay, but sometimes he belabors points (you could say his writing could be more economical, heh-heh). There are times when the book drags, but considering the subject it usually moves along at an acceptable pace. Another fault of the book is that the "undercover economist" persona employed by Harford is alternately forced and poorly developed. I got the feeling it was a gimmick grafted onto the manuscript late in its development.
All in all, The Undercover Economist is a better place to start than Freakonomics for someone interested in learning about economics. However, I don't think either is the best choice. For that, stay tuned (note: the answer is not Freedomnomics -- John R. Lott, Jr. is a tool)...