A reader wrote to ask what he could read that challenged Keynes and his system. On a popular level, there’s Hunter Lewis’ book Where Keynes Went Wrong. Henry Hazlitt’s book The Failure of the “New Economics”: An Analysis of the Keynesian Fallacies goes through and critiques the General Theory line by line. It’s a valuable book and a great achievement, but my own opinion is that it gets so caught up in line-by-line minutiae that the reader never really gets the big-picture critique. Mark Skousen edited a good collection of essays called Dissent on Keynes: A Critical Appraisal of Keynesian Economics, which you can read for free online.
Murray Rothbard wrote the lengthy memo “Spotlight on Keynesian Economics” when he was only 21. Also worth reading are Robert P. Murphy’s “The Critical Flaw in Keynes’s System” and George Reisman’s “Standing Keynesianism on Its Head.”