One of the best introductions: https://forallx.openlogicproject.org/forallxyyc.pdf https://forallx.openlogicproject.org/forallxyyc.pdf Highly recommended. Al

One of the best introductions: https://forallx.openlogicproject.org/forallxyyc.pdf https://forallx.openlogicproject.org/forallxyyc.pdf Highly recommended. Also provides good preparation if you want to go on to study topics like non-standard logic and automated proof systems.