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

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

An Introduction to Number Theory with Cryptography (Textbooks in Mathematics)