For pure theory here are 3 good options:
Per Bylund's Primer: How to think about the economy (easy)
https://mises.org/library/book/how-think-about-economy-primer
Henry Hazlitt's Economics in one lesson (intermediate)
https://www.liberalstudies.ca/wp-content/uploads/2014/11/Economics-in-One-Lesson_2.pdf
Ludwig von Mises' Human Action (hard but great)
https://mises.org/library/book/human-action
A couple of honorable mentions:
I think Rothbard's economic history is also excellent, eg. America's Great Depression
https://mises.org/library/book/americas-great-depression
Finally Bohm-Bawerk's refutations are especially skillful, eg. Karl Marx and the Close of his System
https://mises.org/library/book/karl-marx-and-close-his-system