Sets, Logic, Computation: An Open Introduction to Metalogic (Open Logic Project Textbooks)

Set Theory: An Open Introduction (Open Logic Project Textbooks)

Boxes and Diamonds: An Open Introduction to Modal Logic (Open Project Textbooks)

Incompleteness and Computability: An Open Introduction to Gödel's Theorems (Open Logic Project Textbooks)

Metalogic: An Introduction to the Metatheory of Standard First Order Logic