The Place of Human Action in the Development of Modern Economic Thought" and "The Development of Keynes' Economics: From Marshall to Millennialism," the later being the definitive piece to read if you want to understand Keynes' economic vision.
Money, Sound and Unsound, a monumental collection of work showcasing this treasure trove. The first printing quickly sold out, but now it is available again! This book includes the most important articles Salerno has written on monetary theory and history. Anything Salerno writes is worth reading and his contribution to monetary economics is especially valuable.
Given what passes for analysis of the causes of our Great Recession and the myriad of policy advice that would merely put us back on the road to false prosperity, Salerno's work is more important than ever.