Teach Yourself Logic: A Study Guide [pdf]
I'd like to recommend Logic 2010[1], a application with which you can do logic exercises to learn the natural deduction system from *"Logic: Techniques of Formal Reasoning" by Kalish, Montague, and Mar.[2]The program is officially only for MacOS and Windows, but with a little work it's possible to get it running on Linux, as it's just Java.The exercises are fun, and the program will not only check your work to make sure it's right, but also point out where and sometimes why it's wrong.[1] - https://logiclx.humnet.ucla.edu/Logic/Download[2] - https://www.amazon.com/Logic-Techniques-Reasoning-Donald-Kal...