An economist friend of mine at a conservative "think tank" in Washington, D.C. asked me to recommend 5 to 10 free-market oriented economics books that he could recommend to interns so as to "expand their universes." I quickly came up with 5, but I don't want to bias the discussion.
So please make your recommendations. You could recommend any number of books between 1 and 15. I'll treat it as a Chinese menu before I make my recommendations to him.
UPDATE: The response has been tremendous. Thank you all. Don't let that stop you if you have not commented yet and have something to add. I will send a link to my friend, as well as my own picks. Some of them will be from your lists below.