Forall X
The original Forall X text by P. D. Magnus provides an accessible introduction to formal logic, with exercises and proofs covering propositional and predicate logic. Available with a CC BY 4.0 license.
Open-Access Logic Texts
Below are some open license logic textbooks compatible with this site.
Forall X (Cambridge)
A modification of P. D. Magnus's Forall X text by Tim Button. Also available with a CC BY 4.0 license.
Forall X (Calgary)
A modification of P. D. Magnus and Tim Button's Forall X (Cambridge) by Robert Trueman, Richard Zach, with contributions by J. Robert Loftis and Aaron Thomas-Bolduc. Also available with a CC BY 4.0 license.
An Introduction to Critical Thinking
An open-access critical thinking text by Matthew Van Cleave. This text covers aspects of formal and informal reasoning. Available with a CC BY 4.0 license.