"Economics in One Lesson" by Henry Hazlitt ia another great elementary book on economics and is available online here for free.
Also, "The Concise Encyclopedia of Economics" is available online here
"Price Theory, An Intermediate Text" By David Friedman (Milton Friedman's son) is available online here.
"Cost and Choice: An Inquiry In Economic Theory" by James Buchanan is available here.
"Risk, Uncertainty, and Profit" by Frank Knight is available here.
The first book listed - "Economics in One Lesson" is an easy read and provides examples to make the concepts more accessible. All can be read by anyone unfamiliar with Economics and are great books.