forall x
P.D. Magnus
Open access and open content.
Licensed under Creative Commons Attribution-ShareAlike 3.0.
forall x is an open access introductory textbook in formal logic. It covers translation, proofs, and formal semantics for sentential and predicate logic.
A Primer for Logic and Proof
Jeffry L. Hirst, Holly P. Hirst.
Open access but not open content.
A Primer for Logic and Proof is an open access primer to formal logic.