If you liked Freakonomics, try The Undercover Economist. Subtitled "Exposing why the rich are rich, the poor are poor, and you can never buy a decent used car," this book by Tim Harford seems a bit -- but just a bit -- more hardcore than Levitt's book.
Of course, my metric is how hard one is to read right before I go to sleep compared to the other. It probably wouldn't stand up to the scrutiny of either economist.
In any case, if you'd like to find out how you might avoid self-selecting a higher price for essentially the same items, Harford has the answer: the price of lower prices is eternal vigilance.