I found The Undercover Economist by Tim Harford to be useful, interesting and, unusually for a book on the "dismal science", entertaining. It gave very clear and straightforward explanations on a range of economic subjects including scarcity and its effect on prices, externalities and how to regulate them, international trade and the impact of trade barriers and an explanation of why some countries have grown richer while others remain poor (or get poorer). What gives this book an edge is that the economic explanations are linked to practical everyday issues relevant to most of us as workers, consumers and investors.
While the issues discussed were generally familiar to me, the explanations were helpful in clarifying my thinking on several issues (especially the sections on how to deal with externalities). Many of these are issues of which investors should have at least a basic understanding. For example, property investors should read the chapter on scarcity. One subject that I would like to read more about is game theory - something about which I know very little.
I had one minor gripe - I found the incorrect use of grammar mildly irritating. I must be getting overly pedantic in my old age.
I preferred The Undercover Economist to Freakonomics. While Freakonomics was also an enjoyable and very interesting read, The Undercover Economist was of greater relevance to me as an investor and a consumer.